I've got a CPU block and single GPU block with two 60mm thick rads (420 and 360) with my single D5 pump at setting 1. No problem whatsoever with temps.
I think you could lower the pumps to setting 2, perhaps even 1, depending on your overclocks. Just monitor the temps, if they are within your comfortable zone then no worries
Should not be a problem at all unless you have a bunch of kinks in your tubing, or 4 meters of the stuff. Just watch your temps and if they are acceptable to you, then its good.
very much a suck it and see scenario, assuming you have them in serial, just turn them down and keep an eye on temps, play some games, watch temps, if it's all good, you're gravy.
Protip, try and offset their speed by a tiiiiny amount so they don't sync up their oscillations and create more noise