Hi and welcome!

I don't think I've "met" you here yet. It sounds like you and your kids have been enjoying Little Hearts. And, I'm quite sure that what you're asking about is going on to Beyond Little Hearts, right? The Heart of Dakota programs go...
Little Hands to Heaven (LHTH)
Little Hearts for His Glory (LHFHG)
Beyond Little Hearts for His Glory (BLHFHG)
Bigger Hearts for His Glory (BHFHG)
Preparing Hearts for His Glory (PHFHG)
It took me a while to figure out which was which as people "talked" about them and abbreviated.
Now I haven't used the 2 programs that you're talking about yet... I'm doing LHTH with my 4 yo and going to start Bigger and DITHOR with my 8 yo. BUT, I've learned
a lot from all the great ladies here who are so good at talking through options with everyone. (I'm sure you'll get lots of help!

) I can see a couple of options for you...
1) You could continue to keep them together for the "Learning through History" page and then get the Beyond manual and split them for the "Learning the Basics" part. Your dd could go on in math, add in the light grammar that is there, finish phonics. You could continue to do their read-aloud together, or let them both listen to 2 read-alouds and whoever it was "for" could narrate or answer questions, or separate them. Since you're ahead on the Burgess books, you could get more of them that LHFHG doesn't include maybe?
2) Get the Beyond program and start with you dd and just continue LHFHG with your son.
Option 1 is what I would try if I were you. One other thing that I have learned from Carrie and some books about Charlotte Mason is the value of reading books slowly on purpose. (I may not, probably not, have ever thought of this one on my own...

) It gives the kids a chance to "live" with the books for a longer time which increases what the learn and remember from them. (I may be forgetting other benefits here, but I think that's the general idea.) Anyway, you could try taking the readings slowly, like they're scheduled, too, leaving your kids begging for more!
I look forward to hearing what works for you!

Kathleen