The two options I can think of for replica sets on two boxes would be as follows:

1. 2 VMs on each machine.
2. 2 mongod instances on each machine.

Is either of these better than the other or totally imbecile in comparison to the real solution? 

This is part of my research for a litte side project: <http://bearassbear.blogspot.com/2010/10/idea.html>

Any help/advice/guidance would be greatly appreciated.