However, yes, some issues associated with WordPress are mitigated by the separation; namely, that if the machine B is hijacked, then A is not hijacked right away. As explained above, you are still in deep trouble, but not as deep as you would be if both machines had been the same. "Mitigation" might not be the right term; let's call it "partial damage reduction".
If you want a separation which is effective in the security model as implemented by browsers, then you should separate servers under two distinct domains. For instance, there is
security.blogoverflow.com: that's what you should aim at. Reverse-proxying a vulnerable site is really looking for trouble.