Suggest removing pads and pushing both pistons fully in as you might when fitting new pads - ensure m/cylinder reservoir doesn't overflow though. Then slide in pads and ensure the backing plates of same have enough clearance to move in and out from face of disc as they obviously need to when working. A little Ceratec (anti squeal lube) on the very edge of pad backing plates may help. Thoughts are that outer pad is fouling the caliper slot it works in (maybe due to poor machining) although worth comparing inner and outer slots.
A further possibility assuming the problems doesn't lie with the new disc rotors themselves, is that the brake caliper supports were removed from stub axles and bolted back with some foreign body between mating surfaces, leaving calipers not true to disc rotors. I admit this is a long shot though.
