First years of homeschooling can be tough! I remember having this fairy tale type notion when I began homeschooling that our days would be pure joy-filled from sunup to sundown - you know, birds singing, sun shining, skipping through the tulips loving every single minute of homeschooling, my dc being so incredibly thankful I was their teacher and often thanking me, me imparting vast knowledge in incredibly winsome and entertaining ways... yikes!

Don't get me wrong, we do love homeschooling, but I believe we have not ever had a completely entirely 100% perfect day. It is work - for us and the dc. And we've all butchered things, believe me, so no worries there.

The good news is HOD can be an answer to prayer - a way to spend our days happily with one another - not perfect days, but good days because we have learned to work together and enjoy it.
Well, as far as your dc, it sounds like they place best individually in PHFHG (provided dd makes the gains you are thinking she will by fall), and Beyond Little Hearts. Placement-wise, because they differ greatly in their abilities, the separate guides would individually meet their needs the best.
However, if it is your heart's desire to combine, you could certainly do so. You'd just know that if you chose Bigger Hearts, it would be quite difficult for your ds, and if you chose Beyond it would be quite easy for your dd.

I'd probably go with the latter as you can beef up more successfully (IMO) than tweak down.
If you chose Beyond for the 2 of them, you could beef it up by doing the right side of Bigger Hearts with your dd, using appropriate levels of LA and math for her and having her independently do the Bigger Heart's science box. If you chose Bigger Hearts, you'd have to tweak the writing especially for your ds, as well as many of the assignments, and use the younger LA/math options.
Either way, doing "Drawn into the Heart of Reading" with dd would definitely up the reading instruction level nicely for her.

DITHOR is one of those guides that once you move through a whole unit or 2, it just falls into place and clicks right along. Doing the first 1-2 units is the adjustment period.

I think doing Level 2/3 Instruction/Student Book along with Level 2 book pack sounds like a good fit for dd. Just to clarify, you don't read aloud the DITHOR titles, the dc are doing the reading.
For grammar, she could do R & S English 2 or 3, whichever fit her better. Doing around 2/3 of it orally or on the markerboard, and leaving just 3-5 things for my dc to write each day on paper for R & S, has been a good way to downsize the writing without lessening the learning.

If dd likes to write and ends up doing either Beyond or Bigger, you could easily add "Igniting Your Writing" and do that with her. This would not be hard to do, and it is a good writing program.
As far as the read-alouds, whether you do separate guides with your dc or combine them, you can easily just choose 1 Storytime to do with them. I know this is a lot to think about, and I have prayed for some clarity in the decision for you. May the Lord lead you to the answer that is right for you and your family! I hope something here has helped!
In Christ,
Julie