it's not the kinect themselves doing the tracking work, it's the software libraries. the official openni and nite libraries (probably used in synapse) support multiple users. if synapse can't do it then it's a limitation of synapse, not kinect and its libraries.

Synapse has some nice features and is easy to use, but it does only support one user.

Try OSCeleton for multiple users, or there are other options too depending on system.

i'm a fan of simpleopenni for processing, sending OSC to max

thanks