Christy,
One thing I think all of us go through as our kiddos are getting older and are gaining independence is that our momma's heart longs to stay with our oldest and keep on learning and sharing with that child (as the subject matter just gets so interesting that we don't want to allow that child to head off without us)!

Yet, as our children mature it is so important to allow them to be more mature and to treat them in that manner without having little ears listening in to all of the more grown-up topics and heart issues that come along with that maturity.

It is also so important to give our littler ones the same focused attention that our older ones received, and having an older one moving into the guides from CTC on up allow us to do just that! This is because as our older ones take over more of their own readings, we have more time to spend with just our little ones... bonding and making individual memories with them too.
You will find this to be very true as you head into CTC on up, as these guides are definitely written with the target age range on the guide in mind and are meant to let our kiddos head off more on their own.

This means that we're not writing with an eye toward kiddos beneath the age range, but instead are really targeting the range of ages on the guide both in ability and in maturity.

This makes our guides really fit a narrower age range well, and also make the guides speak to a more clearly defined stage or age that the child is in at that time. So, if you choose to have a young one (beneath the target age range of the guide) listen in, you'll find over time that you're having to read aloud material we never planned to be read aloud (simply to keep your young one in the loop).

You'll also be exposing your child to content that we never intended for a child beneath the age range of the guide to be hearing, simply due to the maturity needed to handle the subject matter in the guide.
Another thing to ponder when looking at placement is how much spiritual maturity and depth of faith a child should have before being exposed to the content in each guide. In my mind, it is very important for a child to have an excellent foundation in the Bible and in what he/she believes before heading into anything involving ancient history and the Reformation!

This means that kiddos beneath the age range of a guide will also not have the spiritual maturity we are looking for them to have in place prior to moving into the historical time period in the guide. This is really something that easily gets overlooked in placement but that makes a huge difference in a child's reaction and understanding of many difficult times in history!
So, while I can understand the pull to want to combine your kiddos for unity in your history study, I also want to encourage you that listening in to an older child's guide leaves out much of the experience we have planned for that child to gain from using a particular guide.

As an example for you to ponder from my own experience, I'll share that my oldest son did Sonlight Core 1 as a first grader. As a part of that core, he was to listen to A Child's History of the World. My oldest was one of those kiddos who read at age 4 and was able to read huge classic chapter books off his dad's shelf (like the unabridged copy of Moby Dick) before he was turning 7. Of course, I took this book away from him mid-way through, and told him daddy's shelf was off-limits, but my oldest son was just one of those amazing readers!
So, as we journeyed through grade 1 of Sonlight, he read more and more of A Child's History of the World himself. He had a great understanding of the world and could really comprehend the readings, so we kept on going. As a couple of years passed with Sonlight, however, I realized that the subject matter just kept getting harder, more violent, and more mature. I began asking whether just because my son "could" read this type of more mature material, whether he really "should" read that type of material. By the time we were in third grade, my philosophy was shifting drastically, and I was realizing that there were many things that required a depth of faith he didn't have (at age 6, or 7, or 8, etc.) to bring to the study of those types of books. I also made a shift away from Sonlight for this very reason. There is much more to reading than simply being able to read and comprehend! Maturity plays a huge role, and even mature kiddos need to grow up to really "take in" what they're reading on a deeper level!
So, my next choice was to use Ambleside Online. We did years 3, 4, 5, and part of 6 in full. While we moved to a more CM approach with Ambleside, and the readings were less lengthy, we still ran into much maturity needed in the readings. Books that would have been a joy for an older reader, were so-so for my older son. He used Ambleside on grade level and had no problems with the level of readings, but in looking back I can see now that reading books like the unabridged Robinson Crusoe as a 4th grader left less of a good impression than they would have left if my son were much older and more mature when he read them (both in age and in his faith).

Through Ambleside, my son read A Child's History of the World (again). What a difference in his understanding now that he had matured several years! Every light bulb in his mind was going off like crazy as the maturity he brought to the reading made a big difference in his understanding. He was making connections all over the place and his thought process was much, much deeper.
With this in mind, as I completed the writing of CTC for my next oldest son, I chose to leave Ambleside and have my oldest son do CTC as a 7th grader. Even though technically the readings were "below" his level by a longshot...what an amazing year he had! He deepened his faith and love for the Lord through his first really Biblical tour through the ancient time period!

He understood and enjoyed what he read so much more than he had with Sonlight or Ambleside and his love for history and reading returned.
The following year I had him do RTR as an 8th grader. Again, I couldn't believe how much the study deepened and matured his faith. It was a turning point in his education and a turning point in my thinking. No longer did I think that he had to be challenged in every area to have a great learning experience. For the first time, I realized how much a mature faith meant to a study of a historical time period!
So in comparison, when we look at having your 8 year old, third grade son do CTC next year, in comparing it to the experience my 7th grade son had, you can imagine my hesitation in ever recommending that option. "Could" you do it, with a lot of tweaking? Probably. "Should" you do it? In my opinion, "could" and "should" are worlds apart, and I wouldn't advise you to do it simply based on whether it "could" be done.
What we discover time and again with HOD, is that those families who pull up a child who is outside of the age range of an HOD guide (simply to combine with an older sibling) often can only make it work in the younger guides. After that (usually by CTC) it becomes next to impossible to do this well, and they eventually either end up splitting their kiddos up and moving the young child back down to a guide where he/she truly fits on his/her own or they end up moving away from HOD. On the other hand, families who choose to combine their kiddos who are actually within the target age range of the guide are able to do this quite successfully. So, it is not that we don't recommend combining, but rather that we don't recommend combining kiddos outside of the age range of the guide.
In all honesty, since another poster mentioned that the board was very "negative" in suggesting that you don't combine a child under the age range with an older child, I would instead say we are looking toward the future and being realistic about whether this is a plan that will work for you well for the long haul.
The wonderful thing about posting on the author's board is that when we advise you, we are looking down the road to the "graduation from high school" finishing line with HOD, and the advice we're giving you is to keep you from burning up your HOD options and leaving you in a pickle!

Simply listening in to an older child's reading is by no means the same as actually "doing" everything that goes with those readings. Pulling a child back a guide is hard to do, and allowing a young one to hear everything an older sibling is reading (without doing any of the skills involved in those readings) is just stealing your thunder for the future when your younger child gets there.

Spiritual maturity and depth of faith play a huge role in the appropriateness of historical subject matter. I wouldn't be in too big of a hurry to grow your young one into an upper guide. He will get there sooner than you'd like already.
Here are some threads to ponder as well:
Why don’t you recommend having children younger than the target age range of the HOD guide simply listen in with the older student’s guide?
viewtopic.php?f=6&t=9341
What would my child be missing out on if I did choose to combine him/her in a guide that doesn’t fit him/her well on the placement chart?
viewtopic.php?f=6&t=9342
How will we be learning as a family if we do separate guides?
viewtopic.php?f=6&t=9343
Blessings,
Carrie