Typically for multiple exposures with equal exposures for each exposure one divides the total exposure by the number of individual exposures to arrive at the exposure for the individual exposures...It isn't as convoluted as what I have just written...For instance, double exposure where the correct exposure is F16 at 1/60 sec each individual exposure would be F 16 at 1/125 second.

In your case I would make my predominant exposure the first exposure and the lighter exposure the second exposure. In the example above I would expose the first exposure at f11 at 1/125 second and the second exposure at F16 1/250 second. This would give you an approximate 2/3 darker exposure -1/3 lighter exposure relationship.

I would plan on using negative film as opposed to transparency film since negative film has broader exposure latitude.