Thanks so much for sharing about your boys! In my heart, I still feel that the best placement would be to separate your kiddos in Bigger Hearts and CTC. However, if you deeply desire to try combining, you would need to go the Bigger Hearts (for the left side and storytime) along with Extensions (for your older son) and add the right side of CTC in the areas my3son's mentioned. From what you've shared, I agree that it wouldn't be wise to try to combine your boys in Preparing Hearts, as in that scenario you'd always be pulling your younger child along in a guide that was on the difficult side for him.
Since your oldest son is beyond the target age range for Bigger Hearts, which is ages 7-9 with extensions for ages 10-11, you'd need to add some of the suggestions we give within this past thread, for beefing up the Bigger Hearts program for your older son: viewtopic.php?f=6&t=640
In this scenario, you also have a wonderful built-in back up plan, as once you get going in the year with Bigger Hearts if you felt that you could handle 2 programs, then it would be easy to bump your oldest son up to do all of CTC (since you'd already own the guide and he'd be working out of it for the science, higher level English, dictation passages, Write with the Best, and doing DITHR Level 4/5 or 6/7/8).
Blessings,
Carrie