Want to plug and play multi midi controllers into Mac

[NewtoMidi]

Just got my first 'studio' up and running. An iMac with garageband, vocals through a mixer and my old Roalnd XP10 as audio in or midi controller through midi to usb interface using GB virtual instruments. First big disappointment was that GB only allows one input at a time, either the input from my mixer or XP10 midi driving softsynth. One of the reason to invest in the Mac was to be able to drive multiple soft synths from more than one midicontoller at the same time. Before I invest in another peice of software, how is this done? Is it simply a matter of running more than one softsynth program and connecting each midi controller to each program? I've read a lot about recording and sequencing but to begin with i simply want to have my Mac act like more than one synth simultaneously so I can play live. Hope someone can help.