cant see it being a fail for having two plates, they might comment on it.. but they must pass/fail as the car is presented.
Years ago I brought some alloys but two were warped. I sent them back and in the mean time ran two of my old alloys/tyres, the tester commented on the different alloys front to back and hinted a question whether I had changed them due to worn tyres, but was open that they have to test as presented, they cant assume or make presumptions.
might actually be worth simply asking them before hand.