Hi Jake,
I too, am interested in using 2 or more instances of Pianoteq. I cannot do this at the moment, as I only have a single 3.2 Ghz cpu. I am looking at quad-core possibilities in a future PC re-build here. I would likely host the pianos inside Reaper, since it is optimized to use 2 quads (8-core) by its developer.
My reasons would be a little on the crude & brute-force side of things. Simply, to pile on the character and resonance to the point where you feel immersed in a wall of very complex sound (like sitting at my 1905 Bell upright with the panels off). Probably, I'd detune the 2 instances by a minute amount - just for subtle warmth.
However, if multiples get too massive & muddy, it may in the end turn out to be enough to use one instance of the up-coming Pianoteq 3.0 (or whichever number it may be called). I'd have to find other uses for a quad core in that case - like convolving spaces etc.
I'm (perhaps unfortunately) instinctively captivated (held at the instrument as a player) by music made from from the stuff of the Universe - matter & energy ("quantum" reality - if you like). -Probably the wrong term. Vibrations as such (banging on old wood pianos in abandoned rooms) hold me for hours (useful for learning theory - ha!, I wish I did more of that). However, I find most sound originating in symbolic (non-real) domain of binary numbers doesn't hold me for long periods of time.
BUT!, -Pianoteq is sufficiently different & powerful (especially possibly in multiples) to trick my muse and hold me to stick with it. I noticed this back with the earliest alphas - the addition of inter-string sympathetics (even if all virtual) was starting to trick my senses and hold me at the keyboard longer than samples. Pianoteq is going to be a great headphone lab learning tool at the very least - and very useful for creating unique sounds! Probably, once my Bell is completely restored, I'll find a way to combine them if I can ever afford midi9 kit etc. The best of both worlds would be sweet. (I may just drive the Bell with a powered speaker as an effect in a loop off of pianoteq too).
Cheers, Jake, and let me know if you get some multiples running. It's definitely worth a try!
-Brian McKinlay.