Synergy would allow you to use one monitor on each and switch between machines. It synchronizes you clipboard between machines which is great and even allows you to drag and drop files between machines. Most people that I know with this issue use Synergy.
I've also setup a single monitor KVM in the past and went dual monitor on one machine, and then used the KVM to swap to the second machine when I need to work on that. That setup works better if you have a primary-secondary setup though.