The MacOS X version of the product is in development. It seems to me that the primary purpose of Multiplicity is to compensate for Windows's poor multitasking capabilities.
I've used a program called teleport, but it's just for Mac. There's also a project on sourceforge called synergy that looks good. It has an OSX package but I haven't tried personally. link Good Luck.
If one of the machines is a laptop or if you have plenty of room and already have monitors. These let you see what's active on both machines at once, while controlling only one machine at time with your single keyboard and mouse.
I've used a program called teleport, but it's just for Mac. There's also a project on sourceforge called synergy that looks good. It has an OSX package but I haven't tried personally. link Good Luck.