The ladies are giving you some wonderful options to ponder.
In your case, based on what you've shared already, I'd lean toward combining both kiddos in Beyond for the left side of the daily plans and for the storytime from the right side of the daily plans. Then, I'd add some of the right side of the Bigger Hearts manual for your 8 year old as needed.I'm thinking it sounds like your 8 year old would need to possibly add the dictation passages from Bigger Hearts (if she's beyond the spelling list 2 in Beyond), the English 2 for grammar (if she's beyond the gentle grammar lessons within Beyond), the Cheerful Cursive or Italic D for cursive, and the Emerging Reader's Set from Beyond/Bigger for reading (once she's done with phonics). Then, you could weigh whether to have your 8 year old use the science from Bigger Hearts or whether you'd just have her do the science that is in Beyond. This plan would work well, since you already have the Bigger Hearts guide.
If you have other questions, ask away!
Blessings,
Carrie