I don't think you are going to be able to do this because it is not a even swap as far as the points needed are concerned. 3 nites in a Category 7 hotel are going to cost you 90,000 points. 7 nites in a Category 7 is 150,000 points. You want 2 rooms for 3 nites each which if you had booked it that way to start with would have run you 180,000 points (2 x 90,000) but you have a certificate that only cost you 150,000 points that you want to use to swap your 7 nites for the two rooms for 3 nites. It ain't going to happen. You will have to send the 7 nite cert back and ask for two certs for 3 nites instead