Surely you don't believe that? I realize that this isn't a perfect reversal but that sounds very odd to me.
It would happen in your model, if there is no perfect overlap between the set of sites in one population and the set of sites in the other population. With two populations, you have more sites. The smartest possible mega-genius is from the mixed population and has + alleles on each site; none of the original populations can have a genius this smart at all.
To see that on less extreme rarity (and approximately for a large number of alleles), write down the ratio of two Gaussians with different means and variances. Simplify. Observe that the ratio of the larger variance Gaussian to the smaller variance Gaussian gets arbitrarily high far from the mean.
Okay but that is an incredibly weak claim - I'm not interested in switching all of the plus alleles on because additivity starts to break down and having an IQ of say 500 isn't particularly meaningful. For any reasonable definition of genius, artificially selecting for the smartest members of a population (what super-Hitler is doing), will increase the number of them.
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.