$60??? I've seen tiny, not-so-well made rabbit cages sell for $100 in petsmart. That's a great size, it's 1 square feet more than the reccomended sixe for a rabbit. Can't wait to see pics!
If there is a gap between matings, both litters will be harmed as one will be premature and the other may be late. The parent's safety and health is of the utmost importance. Most does won't accept service again after being bred, but there are about 20 percent who will and can become impregnated...
Cabbage (especially iceberg) makes all rabbits have runny poop. I think alfalfa hay would be good. I'm glad that you are doing your reasearch before you go ahead and do stuff.
I would never give a rabbit cabbage, but romaine lettuce is good. I'd limit the apples and carrots, and how far into pregnancy is she? You may want to switch her to alfalfa pellets, and switch her back to timothy when the younglings are 8 weeks.
As long as you are sure that they are both females (double and triple check!), and they get along fairly well, then I would try it. But seperate them if they start fighting.
If I were you, I would seperate them all. If you don't at least seperate the males to avoid fighting. And keep the cages at least 2 inches apart, because you don't want them breeding through the wire or picking fur from each other.