You have some good ideas to ponder from the posts here.

I can see some goals emerging from your post and then also from your second post here...
DHT1999 wrote:Julie, combining is not a priority for me. It was the desire to cover American History that caused me to look for something else for him this year. The curriculum I'm using now is supposed to be for multiple levels and at first it seemed like a good thing but now I'm realizing that doing that causes about half of the material in the curriculum to be of little use to either one of the children. I'm sure it's "fine" but we just aren't growing in the areas we need to be growing in. I think using Bigger Hearts with my oldest, even though he's done Preparing (and well!) would accomplish more than what we are doing now.
Prioritizing your goals may really help you choose what's best for you to do. I say this tongue in cheek, for this is something I struggle with myself.

Everything often seems equally important to me, and I can get caught up in the details. My dh is a big picture kind of guy, and so is my friend my sister, so I go to them often to help me see the forest for the trees. You probably are better at prioritizing than I am, but I was just thinking that identifying what's most important and working down from there may help you choose the best fit overall. So, for example, these are the things you've shared thus far...
*you'd like to start BHFHG now and beef it up for your sixth grader
*you'd like more written narrations like last year in PHFHG
*you'd like to work toward having older ds in CtC next year, and younger ds in PHFHG next year, though may need to slow down PHFHG for younger
*you'd like to do American history
*combining is not a priority
*you'd like to grow in the areas your dc need to be growing in, and have found a multiple level curriculum to be of little use to one or the other child
*your oldest did PHFHG well, but BHFHG with ext. would be better than what you're doing now
Let me say that there is no judgment of what decision is right or wrong here - just a matter of what's right for you and what your priorities are, because honestly, HOD can meet those personal goals one way or another.

If doing American History is #1, well then, BHFHG it must be. If growing in the areas your dc need to be growing in is #1, well then your oldest would do CTC and youngest BHFHG (possibly half-speed). If you'd like to start BHFHG now for sure and that's #1, then BHFHG must be the choice. If you want your oldest ds to do more written narrations, then CTC would be the choice. You probably have more goals than you've shared here, and those added to the list will make a difference in your decision as well, but I just thought maybe thinking it through it this way may help you arrive at the final decision you feel best about it. It's just something that has helped me, but please ignore this if it is not helpful to you - I fully realize my shortcomings are not everyone's, and my ways of trying to overcome them are not helpful to everyone for that reason either. I hope something here can help as you think this through!
In Christ,
Julie