Ok, I was intrigued... and think I have disproved the theory... (unless someone can see a flaw in my test)
Part 1, used GertLead and removed mono (i..e play poly) (complexity 5)
Part 2, used AI2 Pad (complexity 3)
played a high note on Part 2 (channel 2) and kept it down.
played many notes on Part 1 (channel 1) , layed my arm across keyboard to get enough notes :o)
Part 2 stopped playing, i.e note was stolen to handle Part 1.
Out of interest (only) i decide to test priority - I then made part 2 a high priority (default is low), did the same test... and this time Part 2 continued to play.
Finally I set both to high priority, and then had first behaviour, Part 1 stolen as expected, as they are then equal priority (both high in this case, both low in previous case)
This means that the note stealing algorithm treats odd and even parts as equals i.e. same resource
I believe this proves the dual DSP are a shared resource across parts, and the odd/even split of DSPs resources is a myth, or at least not true of OS 5.0.3
Of course, my suspicion is the note stealing algorithm is not as simple as above might suggest ... there could be all sorts of 'interesting' considerations it could take into play. e.g. audibility, usage of FX, tail times etc etc.
The above only proves it can steal across odd/even parts so must consider this shared.
(the only 'flaw' in my argument I can see, is if there was a mismatch in the note stealing algorithm and the DSP algorithm but this is highly unlikely and would be a bug)
It would be cool if someone else could verify my above test (or similar) , as i think its pretty important to squash this myth... if we dont get confirmation from Marc/Access
Sorry, I know it doesnt help your situation UltimateOutsider ... as im not sure how this affects your problem.
EDIT:
just did another interesting test with unison... it seems it will steal notes out of a unison first which is quite cool.
i.e. same test as above but changed part 2 to use twin unison, and carefully introduced notes in part 1, and you could hear part 2, first loose on voice in the unison.
im guessing it probably thins all voices first, and then only cuts all notes if absolutely necessary ... though its quite hard to hear (which is guess is the point ;o)
(also be careful, if you up the relative priority, again it will not steal the unison voices either... be nice if that was an 'option')
conditions of test:
5.0.3, TI (1) Keyboard, VCC but using direct audio output (to avoid issues with USB audio)
a TI2, would use same algo, but of course may require more notes to reproduce test.
patches were chosen to have long sustain, and to be sufficiently different hear note stealing.