Yeah, I think OpenXCom allows for more MCDs per maps, but still is limited to how many MCDs per map block?
Having a 8 level mission would be WAY worse than 2x 4 levels though. You'd have to coordinate teams on all levels, and, by "diluting" the XCom team, you would give more time to the aliens (who would themselves be doubled, since you would be fighting both levels at once) to slaughter the civilians.
I think dingo meant fighting either on the top OR the bottom, which could be done by having a 1 level fight which randomly picks either the top or the bottom map.