If I were doing it I would go 136x15. It will give you much better stability and handling in the bumps with minimal push. It should also help out with the extra weight on the back from the second person. The 128x13.5 would be interesting, but may take a little extra to make it fit. The whole suspension is narrower, not just the track. I also know that in deeper snow the tend to trench quite a bit, which wouldn't be good with the extra weight of the RX. I am currently working on putting a firecat suspension (128x13.5) into my SX and have to make my own brackets to take up the extra space.