See this thread
http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=438092#Post438092Accompanied with the image below it should help explain.
sForzando synth plays one sound at time.
But you can load many copies of the sForzando synth at once; each with their own sound.
What you need to know:
Each MIDI track goes to a Port. When you use a default MIDI synth all tracks go to it unless you change this setting.
So assign each track to it's own Port and use another copy of the synth (or even a different synth) on that Port. Problem solved. This is how you use lots of synths in RB.
The image below is of the Right-Click menu of any MIDI track .. it should help figure out how to assign a given synth to a track using the Port
Track-> Port -> synth (either softsynth or hardware)
In the image below I have 16 hardware ports available in my Delta before/above the 16 softsynth ports. The options are there, but you need to set them up for the desired result.
You will likely see your default synth in Port 32 (softsynth 16) on your system.
That's where all MIDI gets assigned when the 'Use Default Synth' box is checked .. but you can change it for each track.