Ableton Forum

Can an Imac be synced to a PC laptop by way of midi? Can 1 audio interface be used or do you have to have two?

http://www.musiclab.com/products/rpl_info.htm
http://nerds.de/en/ipmidi.html

I've used software like this in the past on two Macs running 10.3 (ages ago), and it worked great. No idea about cross/platform though, sorry. Give it a try and let us know how it works.
Currently, OSX supports MIDI over LAN natively! You might experiment with that.

you can do it with two midi interfaces. midi sync is platform agnostic, it's just incoming midi from the outside world.