Multiplexers used as a switch seem to "leak" when power is cut
Two multiplexers are needed to connect the same mother board ground to
  the same mother board positive.


I may be misunderstanding what you've done but it sounds like you're trying to use the multiplexers as if they are physically like this inside:



But, in fact, they're more like this inside:



The point being that the multiplexer is not actually connecting a particular input directly to the output, like a physical switch would do, but rather, a buffered version.

If you want to physically connect the reset switch positive pin to the motherboard ground, you can do that with, for example, an opto-isolator for each motherboard: