Georgette,
The ladies are doing a good job of helping you toss around options for your dilemma. I can see that you'd love to keep your 2 girls together for this year, as it's definitely easier on you teaching-wise to combine them. As long as your 8 year old daughter is doing alright in Preparing and not feeling overwhelmed, for ease of use, you can just keep on going with it this year.
Next year, then, you could drop your 8 year old back to Bigger. It will be a step back in some areas, but that is probably o.k., since this will be a time for her to shine on her own (without big sister in the mix). You can also teach her to use much of the Bigger Hearts guide more independently, because of her experience with Preparing. She will actually fit well within the target age-range for Bigger Hearts which is ages 7-9 with extensions for ages 10-11. Then, when she is finished up with Bigger, just have her move into the Ancient's guide (skipping over Preparing). The Ancients guide is targeted for ages 9-11 with extensions for ages 12-13, so she will fit that age range. If you've trained her well to use the Bigger guide pretty independently, she'll be able to move into the Ancient's guide just fine the following year.
Normally, we would never encourage backward motion, but with your daughter being on the youngest side of the Preparing guide's target range, she could move backward and still fit in the recommended age ranges for the guides.
Blessings,
Carrie