I mostly shoot slides & b/w negs. But as for double/multiple exposures I've only used slides so far. I always went with 1/2 + 1/2 or more generally speaking: 1/n + 1/n + ... + 1/n.

But that depends on the scene. Sometimes I want one exposure to be more "dominant" than the other one, so I do 1/3 + 2/3 or even 1/4 + 3/4...

Or imagine overlapping two scenes, that are horizontally/vertically split... If, say, on the first exposure you have a bright top and a dark bottom, but on the second exposure you have the opposite situation (dark top, bright bottom), then I'd just do 1+1 or to be safer 2/3 + 2/3...