I tried this once, with sisters raised together since birth. Their due dates were a week apart.
The first doe kindled fine...10 babies in a well-made nest lined with fur. The second doe was lazy....she failed to build a good nest, never pulled fur, and eventually had her kits in her sister's nest, on top of the kits already there. A mess....she also had 10 kits....20 is too manyin one box, since you thenhave to worry that some won't get to nurse.
In the end, I had to separate the two does and their litters after the second litter was born...and moving a doe who has recently kindled is not all that spiffy an idea either. New surroundings and all that can really put her off, so she won't tend her litter as well.
You never know tho....rabbits are individuals, IME. Hope it works out for her!