Thanks for sharing more about your dc here, Kimberly!

I do think PHFHG sounds wonderful for your older

, but I am thinking that PHFHG would be way too difficult for a 6 yo

. I would have said the 2 could possibly be combined in Bigger Hearts, but that wouldn't work as your oldest just did that, and I still think Bigger would be too much for a 6 yo anyway. I think it would be best to either continue on with your oldest doing PHFHG and your younger doing Beyond, or backtrack for just a year and combine them in Beyond and beef it up as you can for your older. I want to encourage you that starting with PHFHG, you begin to really reap those years of HOD teaching you did before in the previous guides of BHFHG, Beyond, LHFHG, and LHTH. PHFHG is the mark of a new stage in learning. It is the beginning of more independent learning, and each subsequent HOD guide has more and more independent learning, which is in keeping with CM's motto of once dc are reading well, they need to be doing the reading and the work because they are able.
So, I say this because teaching PHFHG is not like teaching Bigger Hearts. Your teaching time will be less. You could just do the 1 Storytime if you wanted, only doing PHFHG with the 2 of them. Or, you could have your older child just read the PHFHG Storytime independently and do the Beyond Storytime. I did Beyond half-speed and PHFHG a few years back, and enjoyed that very much. You could also have your 6 yo do Beyond half-speed if you wanted as well, doing the left side of the plans on day, and the right side of the plans the next day. This would work just fine too.
If you are leaning toward combining, Beyond would be the best choice. You could have your older read the Bigger Hearts extensions if you went that route. They won't match exactly, but they are still all American History and would be a wonderful way to beef up the history if you want to combine. However, I'm not sure what you would do the following year then. Maybe PHFHG could be done half-speed at that point?
These are just a few thoughts I had, but you will know best. Sometimes it's easier to do separate programs, and sometimes it's easier to combine. You will know what fits your goals the best. I have prayed for you as you have relocating ahead of you too - I know the Lord will provide though. HTH!
In Christ,
Julie