If you put the di before the pumps I cant's see it being a problem, the flow rate for wfp on 2 pumps wouldn't be too much, some ro systems push out a higher volume of water through a single di.. There wouldn't be any pressure there either as it would be before the pumps. You might want to fit a one way valve before the di though to stop air locks from happening inside the di.
Just a thought, what tds are you putting through it, is it ro water or tap? ro water no probs at all, tap probably not but you might want to ask someone who is di only.