Two computers. Three screens - one control?
Teleport is a free application specifically for this purpose. It allows you to use one machine as a "host" with one pair of a mouse and keyboard, and use it to control (over your local network) other machines just by moving your cursor to another computer (similar to how multiple displays work, except this time on separate machines). It also integrates the clipboards of the macs, allowing you to copy/paste between macs.