I've tried to use multiple occurrences of VST-Z3TA+2 in a Sonar X2 song.  But when I do, the patch sound changes slightly.  I can't quite identify it, but similar to dropping an oscillator or effect.  Each VST is on a different MIDI input channel, but that made no difference.  Is one VST dominating or limiting the others in some way?  Is this typical?  Or do I simply have something set incorrectly in Sonar or the Z3TA's?
Otherwise, I love this VST.  Reminds me of playing with an old mini-Moog, but with far more depth and creativity.

I can't say that I have noticed anything like that. Does freezing one of the instances help? To prevent any interaction while freezing you may need to temporarily archive the other instance.

I would add (although I doubt it is the cause of this issue) you may want to check the MIDI playback buffers. The default value is too small for many and adjusting it to 500 or more has solved more than one person's MIDI issues. It can be found in Preferences > MIDI > Playback and Recording "Prepare Using nnn Millisecond Buffers"

Are you by any chance using the same patch on two different instances of Z3TA+2?
If so that could cause a phasing effect that could sound like dropped frequencies, or a lowered perceived volume.
We have done extensive stress testing with Z3TA+2 and SONAR so the issue should be solvable. 
Hope it works out.
Have a good one!

Nope, not using same patch in different instances.  But I found a work around as only 1 of the patches seemed to be changing when I PLAYed the song.  Once I was ready, I simply reloaded the patch, then Bounced it to audio and archived the VST track.  Also found sometimes Sonar did not save the correct patch upon exit.  Not regularly, but at least on two occassions.  (Savings as normal cwp, not MIDI file.) 
I feel uneasy at the Strangeness of non-repeatability in software.