I've seen this kind of consolidation for "foreign" brands (French + Korean/Japanese), but I think most BMW (+ Mini) and MB (+ Smart) dealers are still pretty exclusive, as well as VW (+ Skoda + Audi), which all belong to the same umbrella company. A dealer chain can switch OEMs, but it's not so much a switch as a complete tear down and rebuild of the business. Sometimes even the dealer's buildings reflect the OEM's brand, not just through signage and colors, but architectonically.