This is done by right-clicking the track name and selecting 'port'.

Then scroll down until you see one of the empty DXi synth tracks. They are ports 17-32 I believe.
After assigning the track to that port, you assign a DXi synth to the track using the little arrow beside the track number, and in the top box selecting an available (installed) DXi or VSTi synth. If the synth isn't in the list scroll to the bottm and select 'add new DXi synth', at which point you'll be asked to browse to the .dll file for that synth (the actual sile that runs the synth)

All other tracks will then run on the default synth, but that track will send sound through a different synth. You can use up to 16 different synths if you're system is up to it.

You can also assign other tracks to that second synth by selecting that port for a given track... So there are many variations to have fun with. I did a song once just trying a bunch of different DXi/VSTi synths out at the same time .. I should try to dig that one up; it was kinda fun to listen to in the end.

Make your sound your own!
.. I do not work here, but the benefits are still awesome