I was in a double-cast show when I was in 8th grade. If someone from the other cast was absent, we stepped in, and when everyone was present, the opposite cast sat in the house and watched. The director tried to alternate daily for us, though I think my cast got less rehearsal, since we were the "B" cast.
I think it's a matter of keeping the second cast together and paying attention during rehearsals. We do this quite often in ballet - and at one point I did a ballet with about 40 kids, all double cast, so 80 total. We rehearsed scenes with one cast shadowing the cast on the floor, then we'd switch casts when the scene was complete. Come tech time, we ran the show twice, though one cast didn't get the costumes. Granted, in my situation, since this was through our ballet school, I had to be kept constantly up-to-date with who was dropping out and who would either be replaced, or if the role would be become a single-person role. 8 per cast sounds a lot more manageable.