Welcome to the HOD Board!

Thanks for sharing about each of your precious dc, and I'm so glad you have found HOD. It is such a blessing in our home, and I'm sure it will be in yours too!

We did begin Beyond with my 5.5 yo ds, and he did well, but needed some time to grow into it. We started half-speed for a bit, and then did it 4 days a week instead of 5 the rest of it, so it took us about a little over a year to get through it all. That extra time was a big help to him. I also only had him copy 1 sentence of the poetry each day, and I had him do "A Reason for Handwriting A", as I felt he still needed to work on his handwriting. He grew into copying all of the poem partway into the year. He was reading well and halfway through the Emerging Reader's Set when we started Beyond, but it is just fine for a child to be doing phonics during Beyond, and is in fact, one of the reading options listed in Beyond's plans. It would just be very important that by the time you get to Bigger Hearts with the 2 of them that his reading and writing skills were quite solid. We've started Bigger Hearts with my almost 7 yo half-speed and will take the rest of this school year and all of next school year to finish it (doing it full-speed next year). We're doing this so he will be in the mid-age range in PHFHG. So, I think as long as you are able to adjust your pacing as necessary, and as long as your youngest ds's reading and writing come along enough to do Bigger Hearts later (even half-speed if need be), than it would be fine.
Another option - you could easily have your 5.5 yo do the right side of LHFHG but the left side of Beyond with your older. That would be very easy to do, and would be easy to continue to do as the 2 of them move through guides. Or still one more option, you could certainly separate them and have your oldest do Beyond, and your younger do LHFHG - which from what you've shared sounds like the best placement for them individually, but may not be the best placement for your family goals as a whole. So, based on your desire to combine, I think you could definitely do Beyond with the 2 of them but slow down the pace to possibly 4 days a week, taking a tad longer to do it, and also really working on your younger's reading and writing skills this year to adequately prepare him for BHFHG and to be able to successfully combine him in the future with older dd.

I think if I were you, I'd start Beyond with the 2 of them, knowing you have to let ds grow into it, possibly having him do the right side of LHFHG instead (or have that as a backup option if you want to try the right side of Beyond with him first), and see if ds can hang with it. I think if he can listen to read alouds with fewer pictures and can grow in his reading/writing skills, he'll do terrific!
These are all just ideas to consider - you will certainly know what clicks the best for you and your dc! Beyond is great fun to do, and such a joy to teach, but so is LHFHG - so that can always be a backup option. Keep asking questions here until you get your placement fine-tuned so you have a peace about it! HTH - and I look forward to getting to know you here!:D
In Christ,
Julie