I have posted that I am combining my kids (ds=5, dd=7.5) on the left side of Beyond. My ds finished a traditional textbook K year and is easily an emerging reader. My dd just finished 2nd grade. We just had a baby in April so I want to combine where I can for that reason. More importantly, I want to enjoy studying subjects together which is why I opted for HOD. But I realized today my plan may need a rework. Your guidance is requested.
My ds will have no problem with Beyond as is and is a great place for him.
My dd needs the right side beefed up so I was planning on doing the right side of Bigger. I didn't realize this might not be enough for her until today. I was planning on her doing the R&S 2nd grade book as her language arts but low and behold after a year of 2nd grade with another curriculum, she tested out of ALL of R&S 2nd grade with flying colors (took their tests and scored 90% and above on all tests, even after a 3 month summer break). The logical thing is to just have her start working on R&S 3rd, but I see on the Preparing pages, there is more to that level's language arts than just the R&S work. Hmmm.... You may be wondering about spelling. The answer is I will be having her start off with with level 2 dictation. She will be doing DITHR 2/3 for reading. If she needs practice anywhere it is writing and spelling. Gracious me. Do I have her do the right side of Preparing with the left side of Beyond? That is quite a spread.
What do you recommend knowing I really want to keep our history, bible, science (b/c it relates to history) and storytime together? You know better than anyone what kind of hole I could be digging, potential pitfalls, etc. I don't want to push my ds too hard but I still need to keep my bright dd stimulated.
I look forward to hearing back from you.
Laurie Beth
Footnote: She just had to write a short paragraph for me. While she can remember to capitalize her sentences and proper nouns in a test, she "forgets" when writing a paragraph. Her first draft was 1 long run-on sentence instead of 3-4 individual sentences. In addition, her spelling left something to be desired. The words misspelled included because, bird, fluffy, and chirps. (We have a cat that chirps like a bird instead of meows. It is hilarious!). As I said, she needs practice writing and spelling. Suggestions??
I think I'd keep her with ds for the left side of Beyond and do the right side of Bigger with her. Beginning DITHOR 2/3 will challenge her, as will the dictation. The dictation will help her spell well within her writing - it has done wonders for my ds in this area. But it takes awhile, so just don't be expecting it to happen overnight. 
I like your idea of doing R & S 3 over 2 years. I think that would work out well. Then, you'll be on a nice pace to combine them, while still meeting your dd's needs nicely. HTH!