Why can't I use PLA with ABS in a dual extruding 3D printer?
First of all: PLA is not water soluable.

Second: You want the melting points of the plastics to be as close as possible. PLA is printed between 180 and 200 °C. ABS is printed at 220 to 240 °C.

PVA on the other hand is water soluable and is printed in the same temperature range as ABS.

The setup is dependant on your slicer: you need to select one extruder as being the support structure one, the other as the main body one. Then you assign the filaments to them. Without knowing your slicer, I can't asnwer this better.