As a rule of thumb, stopping down one step (or halving the time) for 2 exposures to 1 frame is good enough. With negative film, it will always give you a decent exposure, but some combination of exposures may need less compensation. Of course, you can also underexpose one by 1.5 stops and the other by 0.5, if you want one picture to be more dominant. This is not mathematically precise and applicable in all situations, but good enough, even for color slide film. For multiple exposures (three or more), half the number of exposures is the number of steps to stop down... 3 exposures is 1.5 stops, 4 exposures is 2 stops and so on.