The bluish one actually isn't an M. rosenbergii. It looks like an M. tenellum (which is a native of Ecuador, the Pacific-watershed sister to M. acanthurus) or something in that complex. Not likely that M. rosenbergii would be able to hybridize with any New World species. If the smaller one comes from West Africa, then yeah, it's quite possibly M. dux. This genus contains over 230 described species (I'd estimate more than 300 total), many of which look pretty similar, so eyeballing isn't always a good bet ID-wise.