FXT is PC-only at the moment, but what it does is stream audio and MIDI over ethernet from one machine into another. That's a substitute for a sound card or a mixer - a great thing but a totally different subject.

The reasons you can't use EXS24 MK II (as opposed to the player, which doesn't stream) on multiple machines practically are that a) you'd have to buy a whole new Logic Pro to do that, and b) even if you were to do that, the workaround to keeping multiple Audioinstrument channels open for live input involves inserting I/O plug-ins on all the tracks you want open. That eats up too much CPU.