Kim,
It is good to know going in that both LHFHG and Bigger will require you to be teaching almost everything. However, don't despair, because both of these guides are great training for what is to come! So, in essence, Bigger is your biggest teaching year, because you have to be present to do almost all of it. It pays big dividends as you move on to Preparing though!
As you look at your schedule, one thing that I would caution you against doing is letting your LHFHG child listen to all of Bigger's history or science. This is because you will be stealing your thunder as CM would say that narrating from a reading heard only one time makes the reading much more interesting (and the habit of attention much more important)!
If your child is just playing nearby, it really isn't a big deal. However, just be sure the younger child knows he/she is listening only, not responding.
The same is true for science. Sometimes I let my littles jump in and participate and sometimes not. I do make sure not to think of ways though to be sure to include the younger child. I just try to keep in mind that this is the older child's history and science, and if it works to include the little one right along as written it's fine, however I'm not coming up with ways to make it fit the younger child.
If you want to have your two kiddos listen to each other's storytimes that would work fine, because when you get to Bigger with your next one you can simply choose a different set of storytime books to read aloud. Perhaps your Bigger child could be doing cursive while you are reading aloud from the LHFHG storytime. Just be sure that the LHFHG child is the one who answers the LHFHG questions and the follow-ups in the guide. Be sure your Bigger child knows it is just a listening opportunity, not an answering opportunity. The same is true for when it is the Bigger child's storytime. Perhaps, you could read that one aloud while your kiddos are eating lunch, or else right after lunch. This has worked well at our house in the past, because it is a time when we are all together anyway.
They hymn from Bigger and the Hide Em in Your Heart music from LHFHG is another thing that works well together.
The older child can also help out with the Rhymes in Motion, and if you wanted your younger one to listen to the older's poetry while playing nearby that works fine too.
Math is something we are able to do around the table, as is phonics and Drawn into the Heart of Reading. I plan those round-table style right now with my youngest 2. I go over the older one's math and get that child going, while the LHFHG child does either Do It Carefully or Handwriting K. Then, I teach the younger one's math lesson and get that child going, while keeping an eye on the older child's math. When the older child finishes, he moves onto DITHR. I do any lesson and send that child off to read nearby, while I do phonics with the little one.
My kiddos know to be patient during this time, and it can be a little hectic, but it does work well. We've done some version of this around the table with all of our kiddos through the years in various combinations.
To keep your day from going to long, I'd work to do the left side of LHFHG in one 30 min chunk with your youngest child without interruption if possible. To do this, be sure that any other kiddos you have are occupied for 30 min. with something that they will not come from to interrupt you. It can be educational computer or reading a book or playing with a younger sibling or something to that effect. It does not have to be school-related for that 30 min. As you can see, with what I've shared thus far, after the 30 min. left side for LHFHG, your LHFHG child will be done (if you did motor skills, math, and phonics round table style and storytime while the older does cursive).
As far as your Bigger child goes, it can also be helpful to have a 30 min. uninterrupted chunk of time where you move through 3 boxes on the left side of that guide as well (probably Reading About History, Bible, and the bottom left rotating box). The poetry and hymns and storytime you would be doing along with your other child. DITHR and math you'd be doing round table style. This leaves you grammar, dictation, and science to finish some other time in your schedule.
I'd probably split them and try to grammar and dictation first thing while the younger one is still sleeping, getting ready for school, or eating. Then, I'd probably do science last, after reading storytime to everybody together. But, you'll find your own fit.
These are just suggestions to get you thinking of possibilities that may work for you.
Each family is so different, so what works for one may not work for another. Hopefully, this dialogue will help you find the right fit for you.
You can do this!
Blessings,
Carrie
. Let them listen in if they want to and you don't mind. It is really YOUR choice. 
The readings in LH are short and will be fun for your 3rd grader to enjoy. The storytime for Bigger can be storytime for everybody. The history and science readings may or may not interest your first grader and are worth reading more than once over the years.