Upstream sensor, (pre-cat) on bank one is B1S1. Upstream sensor (pre-cat) bank two is B2S1. The typical configuration with two post cats are numbered B1S2 and B2S2. A simpler way to do what I think you want to do would be leave B1S1 and B2S1 alone and just delete the post cat (sensor 2) data from your existing tune.