That's probably as close and as accurate as you're going to get. GM, Toyota, Nissan, Honda and Hyundai are the big ones that aligned with XM at the beginning, especially GM being an initial investor/partner. Ford, Daimler Chrysler, BMW, VW were big ones for Sirius. For the most part, as the auto industry changed and some of the foreign auto makers lost the backing of the domestics, things pretty much stayed the same. Mazda, Volo and Jag stuck with Sirius from the Ford days, Mercedes and Chrysler each also stuck with Sirius from the DCX days. Hyundai is an exception. Hyundai uses XM hardware and Kia uses Sirius.
Yes, my understanding is Sirius XM hardware can receive signal from both sets of satellites. Although I'm not really sure how that would work and which system channel tuning is defaulted to. In the Drop down if you choose a 2015 Ram 1500, it says SiriusXM Satellite Radio underneath the Audio Programming section. If you select a 2015 Chrysler Town & Country it says Sirius Satellite Radio.