I agree that a rudimentary evolutionary algorithm probably wouldn't work since they often don't take into account multimodality. There are global stochastic algorithms that take into account this sort of multi modality. Most particle swarm framworks could handle this sort of bipartite nature with a more complicated fitness function. Differential Evolution frameworks in R also allow you to do a post operation on each candidate, which include something like rejecting it because it does not seem to lie in one of the two modes.