I tried to find evidence as to whether this would work before I bought the SIR-PNR2 (I already had the XM NavTraffic box) but came up blank. So, I just went ahead and bought it and installed it.
The XM runs on the standard XM input and the Sirius runs on EXT-1. Predictably, the features of the SIR-PNR2 are heavily limited on the N2, but it works.
So there you are, confirmation that you can run both sat services at once on an N2.