Next September, my daughter will be 9 and in 4th grade. My son will be 6 and in 1st grade. I've spent considerable time looking at the placement chart, and ideally me son would be doing Beyond and my daughter would be doing Preparing. But.... I don't know if I'm up to doing two separate programs. Would it be ok to put both of them in Bigger? My daughter is advanced in her language arts and math skills. She loves to read and reads independently very well. For grammar this year she is doing First Language Lessons, Level 3. I'd like to see her work on more formal writing skills, and it looks like there is more of that in Preparing and not so much in Bigger. My son is finishing up Kindergarten now and is right where he should be. He's reading easy readers independently and his handwriting is atrocious.
He has a good attention span for sitting and listening to me read to him, but very limited attention span when it comes to cutting, coloring, holding a pencil, really anything that requires focus and fine motor coordination, excluding Legos of course! I also have a three year old and one year old, so it can get a little crazy around here. I don't have a lot of uninterrupted time for sitting and reading with the older two. Combining the two older kids and doing one program would cut back on the time I am reading out loud to them - right? Is Bigger going to be enough for my daughter? Will it be too much for my son? I'm concerned that I will misplace both of them by putting them in the same program! If doing separate programs is really what will make the program successful for us, then I'm sure some of you wonderful ladies will help me figure out how to do just that!
Doing both of them takes me 3 hours, max... and that's with adding in my 5yo who is doing LHFHG half speed. 
As another poster said, while one your PHFHG is finishing independent work, you can teach Beyond. I'd plan on adding 20 minutes each day for your math, as that substitution takes longer. We are doing RevtoRev right now, and it has a fair amount of American history in it. However, you could have your dd read the two Eggleston books this summer for some exposure to American history. They are excellent books! We are loving PHFHG right now. It is such a neat guide. We enjoyed Beyond too - I think this sounds like a good plan for your dc. HOD continues to be an incredible blessing for us. May it be a blessing in your home as well. HTH!