I've found an old band member a distance away who also has X2 Producer. If we both have different add-ons (Waves One Knob, Strum Acoustic, Garritan or other effects etc.), would bounce to clips or some other way allow the other party to work with what you have created. Or would we need to stick to the boxed X2 or have the same add-ons?  

I haven't tried that before, but my guess (if you want to keep sending the files back and forth) is that you could keep everything with non destructive techniques.

For example, for tracks where you know the other person doesn't have the soft synth you could freeze the track. Then save the audio from the frozen track to a separate audio track (so you have all three) - the frozen soft synth track (set to mute) the frozen instrument track (muted) and the unmuted audio track.

The receiving guy could hear the audio track and would just leave the frozen/muted stuff alone. 

When you receive the files back, if you need to alter the soft synth track you could unfreeze it and still have all the original settings.

Same kind of idea for a cloned / copy track where you bake in an effect that the other guy may not have so he can listen to it with the effect(s) on, but you have the source track preserved and muted.

That's what occurs to me when reading your question.