I think guitarman is referring to the “hold pedal” function offered in the FTP software host. Essentially, it is a Sostenuto pedal function that allows you to sustain notes played before the pedal was pushed but play new notes that are not sustained. Implemented for playing leads, FTP allows switching or stacking sounds simultaneously.
@ guitarman try using the “midiSostenuto” plugin available here on the GP website.. I have one of these inserted in nearly every rackspace and have it mapped to a button on my GuitarWing so that I always have it available to me.
As far as simultaneously switching to an alternate sound at the same time you press the switch, there a re several ways to accomplish this in GP. Conceptually, the simplest way would be to map the same controller (that activates the Sostenuto function) to a second widget that activates another plugin chain which provides an alternate sound. This would allow you to just A/B the “signal” paths or stack the new chain on top of the first.
Hope that helps!