1999 megane scenic rear brake discs help

Hi all a bit of a silly question our megane scenic 1.6 16V has failed MOT on a few bits the other half loves the car but we are stuck the car has failed on o/s rear wheel bearing rough when rotated we have just spent £150 on other bits now money it getting tight and priced up rear bearings and renault in the fantastic ways decided to incorporate the bearing within the disc so the cheapest cost of the 2x rear discs £140. The question I wanted to ask is do both rear discs need to be replaced for MOT or can you change just the one that has offending bearing hope someone can shed some light thanks in advance

1999 megane scenic rear brake discs help