You really wouldn't want to run both at the same time....perhaps consider a micro-switch to activate the propane shot at or near WOT, and then have another micro-switch that activates when the pedal returns to full rest.

Propane for power and/or cooler EGT's, and water to wash down the hot components, cracking off any carbon/soot.

Water will knock down EGT's for sure, but IMHO, not a good idea at or near WOT....

 

Water is only for WOT, its entire point is to cool the charge air.