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.

Yes you can run both on any avic. And being an ext1 source will as you said heavily limit the display of sirius. If you read any of the pioneer/sirius bashing threads, you'll get an idea of how bad the interface is.