My only experience with this comes from:
1) accidental double exposures; or
2) the sort of cheesy things we used to do with wedding photos - think bride and groom in a wine glass.
For the latter, it was important to compose both photos with the other photo in mind. You want lighter details in one image to be juxtaposed with shadowed areas in the other. Otherwise they tend to block each other out. As the individual important areas didn't overlap, they would tend to need full exposure, or something close to it (like 1/3 of a stop less).
If you cut each exposure in half, only those areas where both images offer mid-tones will end up receiving the right exposure. Where shadows coincide, the result will be seriously under-exposed. The same may be the case for the highlights as well.
This sort of thing calls out for careful pre-visualization (as Minor White would have said).