If you are using a “regular” MIDI track, routed to the VSTi Rack, rather than using a dedicated Instrument track, then you can use a MIDI Send on that MIDI track, to whatever other MIDI instrument (or MIDI port) you wish. There are four MIDI Sends available.
(I was really hoping that that Cubase 7.5 would bring along MIDI Sends from Instrument Tracks also, but, alas, not yet 
).