I made a weak claim (all sites) to make it easier for you to see how that works within your own additive model. Of course, you don't have to have plus alleles on all locations for a genius to be more common in the mixed population than in the original populations.

For any reasonable definition of genius (someone with an IQ of 160+), artificially selecting for the smartest members of a population (what super-Hitler is doing), will increase the number of them.

This would depend on the population sizes involved, number of locations, and overlap between locations.