You could also consider making two separate negatives and printing one over the other on the same sheet of paper. That way you could determine the exposure and contrast as well as any dodging or burning you may want. Jerry Uelsmann printed five or more negatives (using multiple enlargers) to make one image. If using one enlarger you will have to take the paper out after the first exposure to switch and focus the second so make some indication with a pencil on the back of the paper to place it correctly back in the easel.

I have printed four separate images on one paper using masks. Time consuming but it worked. Lucien Clergue did many in camera double exposures on 35mm with the exposures from different locations and times. I don't know how he registered the film after rewinding but I know he does it because years ago he photographed a model in my backyard over photographs of paintings taken in museums using as I recall a Minolta 370 and no tripod.

http://www.jeffreyglasser.com/