ah, seei have no idea about MDDs, i dont use them. but didnt you state above that the processor from the xserve would be on top because it has more?
... more than any other single CPU for the MDD.
that it doesnt support OS9 by default.
Remove the original CPU, place the 1.42 and restart.
Thas't what I have done with a MDD (and with Mac Os 9, of course)
... so simple and trouble less.
might be something which can keep people away from using such a configuration. you know how hard it is to unattach a IDE disk´s power cable!
May be you have some confussion with Uninorth 1.x G4 machines (from Sawtooth to QuickSilver)
About MDD's I hope this can help: