There is a VSTi called MIDImonitor that, along with showing you the actual MIDI data coming into it, lets you enable MIDI out. Once this is enabled, MIDImonitor's MIDI out can then be selected as a MIDI source for other soft synths.
This way, more than one synth can be set to have MIDImonitor's MIDI out as that synths source MIDI. The actual MIDI data that you wish both soft synths to respond to can be placed in the MIDImonitors track.
I did a simple test with three Simple Instrument tracks
1. MIDImonitor - with its MIDI out enabled - echo on, so I can play with my MIDI controller
2. Rapture Pro - with its MIDI source set to MIDImonitor's MIDI out - echo on - so Rapture Pro responds to MIDImonitor's MIDI out Data
3. Truepianos - with its MIDI source set to MIDImonitor's MIDI out - echo on - so Truepianos responds to MIDImonitor's MIDI Out Data
When I play the MIDI contrller, the synths I hear are Rapture Pro and Truepianos simultaneously. The sounds cease when I turn echo off on track 1 only
I can even record MIDI data on the MIDImonitor track only. When I play these back, I hear are Rapture Pro and Truepianos simultaneously - yet there is only one copy of the MIDI data - which is in the MIDImonitor track.
MIDIMonitor can be download from here
http://thepiz.org/plugins/?p=midiMonitorHope that helps