Hi Jodi, and welcome to the HOD Board!

We have lots of ladies that combine and love HOD, and lots of ladies that don't and love HOD. If you were going to place your 5 yo alone, where would she place? Would it be LHFHG? Is she turning 6 soon? If you were going to just place your ds on his own, would he place in Bigger Hearts? Since there is 3+ years between your 2 dc, you're on the verge of whether combining will work well or not as well, to be honest. I can totally understand you wanting to teach 1 program, especially to begin with, so you can totally do that with Beyond with the 2 of them if you want.

You just would know going into it that it may be tough for the 5 yo to do it, and possibly too easy for the 8 yo. Whether it's HOD or another program, this would be true with those ages, as what 5 yo can do and what an 8 yo can do can vary greatly. However, sometimes that's worth it to be happily using one program.

Just because you start off combining, that doesn't mean you can't separate them later, and vice versa. So I think you have 2 options ahead of you from what you've shared...
Option 1:
Combine the 2 in Beyond for the left side of the plans and Storytime from the right. Then, you could and have your dd do the right side of Beyond for her LA and math. But for ds, you could have him do the right side of Bigger Hearts (minus the Storytime). This would give him dictation and hands-on math lessons too.
Option 2:
Your ds could do Bigger Hearts, and your dd could do LHFHG or Beyond, wherever she best placed.
I think I'd be inclined to do Option 1. You are leaning toward wanting to do 1 program and your dc are just coming out of ps, so this seems to make sense. Beyond is a wonderful program, and I bet you'd all enjoy it! Beyond would give your ds an introduction to the parts of speech for grammar, and let him grow in his LA/math skills. You can give him a math placement test at the Singapore website to see where it's best to start him. I'd start him wherever he places, as it's pretty accurate:
www.singaporemath.com
I'd say it depends how the 2 work together and how big brother will feel being paired with younger sis', too. You may want to read this thread where ladies share why they do or don't combine, as there's not really one right answer to that question, but more just personal preference:
viewtopic.php?f=6&t=5290
I know others will share their thoughts here too, and we'll all chat through this until you have a plan that fits your goals and needs the best. Whatever you end up deciding to do for HOD guides, they are all outstanding!
In Christ,
Julie