Found this in Wikipedia, under Sirius and XM Merger:
Receivers
Currently, XM and Sirius use different compression and conditional access systems, currently making their receivers incompatible with each other's service, though both companies have guaranteed that existing receivers will be able to receive content from both services after a merger.
Executives from both companies have stated that XM and Sirius will continue to operate as separate services. Buying another radio to receive programming from the other service will be optional.[25][26][6] However, a unified receiver that can receive programming from both networks would allow subscribers to listen to the entire range of channels. Interoperable Technologies, a joint venture of both companies, was formed in 2003 and announced such a receiver in 2006. Mel Karmazin has confirmed the existence and operation of the radio, "We have one. It's in my office." Karmazin's dual system radio can expect competitors--according to tech-site Gizmodo, Onkyo is entering the interoperable radio arena.[27]
Okay so it says they're incompatible, but that they do have a receiver that can process both. But it looks like they're gonna try to sell them both as separate company products still. So if one is say $10 a month you could probably get both at a "discount" of like $15 to $17, I guess. So unfortunately it doesn't look like they plan to let you just get both for having paid for one. Figures... if that's the case I'll try to swich to Sirius and ditch the XM. It looks a little confusing and contradictory, though.
