We've decided to go with HOD again next year after a year off!

Here is our situation:
4 kiddos, with #5 due in August. 3 oldest will be in HOD, one toddler, and one newborn. We just moved to a new town/house and are settling into a new life. My oldest (11 yo boy) has generally struggled with schoolwork and so places lower in the guides. I will be putting him in Preparing with extensions for reading. My second (9yo girl) is pretty much on track for a 3rd/4th grader. I am debating between combining her with my 11 yo in Preparing or placing her in Bigger on her own. My 3rd (5-6 yo boy) will be starting LHFHG as a kindergartner.
My question: to place my two oldest together in Preparing or not? Or to place my oldest in Preparing with extensions and my 2nd in Bigger with possible extensions.
My pros for combining:
1. Easier teaching load for me. I will have a K, toddler, and newborn. I would love to have the two oldest start to work independently together on some subjects.
2. Two oldest generally get along really well. They are usually good playmates and best friends. It could be fun for them to study the same thing together.
3. my daughter will be on track for finishing guides.
4. I LOVE the four day plan. We tend to get behind and I have some other subjects I would like to pursue on the 5th day if possible.
My cons for combining:
1. possible competition between two oldest. I don't want them to have to compare their work all year. My 2nd doesn't like it when my oldest finishes the projects first because she doesn't want to see the end result before she is done. My oldest is 2 years older then my 2nd and I don't want him to feel less smart if she can easily keep up. I am planning on giving him the extensions to read anyways so he will be doing something extra than she will, which could help to make him feel older, it could also make him resent his higher work load, who knows?
2. My second is only halfway through her cheerful cursive book. I think I could tweak the assignments until she catches up, but it might be better for her to place in Bigger and concentrate on really mastering cursive. She has not mastered it yet.
3. Looking ahead, I don't know if it would be better to keep the oldest two separate so they can stay in their own guides till the end or keep them in the same guide all the way through.
4. If my oldest two are combined and stay combined all the way through high school, my 3rd child will always be the only one without a school buddy, since my youngest two will also be two years apart from each other. My 3rd is 3 years younger than his older sister and 4 years older than his younger sister, so truly a middle child.
My pros for separating:
1. Give oldest two some independence from each other.
2. It is possible that the work load in Bigger would fit my daughter better. I have the Bigger guide, but not Preparing, so I can't completely compare them.
3.They don't have to share reading/looking at a guide.I've heard that can create sibling battles

My cons for separating:
1. I don't know if I can keep up three guides...

2. I've heard not to run Bigger and Preparing at the same time, and am I a crazy woman asking for burnout if I am adding a K, a toddler and newborn to the two guides??
3. How do I figure out the 4 day and 5 day difference between Bigger and Preparing? I had some ideas about additional subjects we could study on the 5th day if they were on a 4 day schedule together (or just use that day to catch up if needed), but having them with different week lengths is hard for me to figure out. I don't want my daughter to only do four days and take a long time to get through the guide.
Last thoughts about my 9yo DD and her level. She is a great reader, but can be picky about chapter books that seem too long. All the spelling dictation passages in Bigger are too easy for her. She is working on "Spelling Wisdom" by Simply Charlotte Mason. She has mastered manuscript, but is only halfway through cheerful cursive. She is still working on "Climbing to Good English - 3rd grade" grammar. (only 2-3 units in). She will be starting level 3 Math Mammoth (I think, she needs to take the placement test).
Any advice or thoughts?
thanks!!
sara