I'm anal about stuff like this and probably need to let it go

Based on the placement chart, my just-turned-6 dd seems to place best in Beyond, and 8 yo ds places better in Bigger. Truthfully, I would rather place 6 yo dd in Little because she's half kindergarten level, half 1st grade. I'm thinking Little would be a better fit based on the books offered for the program, etc. And honestly, based on the books for the Beyond curriculum, I think DS would be better there. So despite the placement chart saying the kids need to go up a level, I'm thinking they need to go down a level. What are the experiences of those who have gone against the grain and gone down a level? Were you right and did everything go OK, or were your children not challenged in the lower level curriculum?
Also,l how do you teach two curriculums at once, and how much additional time does it add to the day?