IMO you're better of using a stock AD20 with the PS2 for two (maybe three) reasons:
the PS2 outputs via XLR connectors, so with the mod-AD20 you'll need some adapters to convert the XLRs to the stereo mini; with the stock AD20, you won't need any adapters; fewer adapters = fewer potential points of failure, fewer things to forget on your way out the door, etc.
you'll go through batteries faster on the mod-AD20 because even though you're not using the 9v bias power to power your mics, I think the bias power is "always on"
If I were you, I'd confirm with someone (SP directly is probably the best bet) whether or not running the 9v bias power from the mod-AD20 into the PS-2 will pose any problems; I don't think so, but I'm not a technical expert
all of the above is not correct
The mod-AD20 has the orginal xlr input still intact and operational.
The battery will not run down faster as the mod-ad20 has an extra 9v battery attached to it for 9v power.
The bias power does not pose any problems becasue it is only active in the 1/8" mini plug, not the xlrs