I have read conflicting reports. Some saying that OS X (10.1 and earlier) is, at the very least, mildly Dual 604e aware. I have also read other reports that have said OS X has ZERO awareness of pre-G4 MP configs.
Basically, if you have the Dual S900 right now, load up 10.1 with XPF and give it a shot. Let us know how it goes.