I was thinking because there have been a lot of instances of two separate Countries forming unions, if two Dynasties would do something similar, or if one dynasty was trying to assert some kind of heritage/authority. Fictional ex. House of Premyslid-Hohenzollern
Well, the House of Habsburg went extinct in the 1740s, with Maria Theresa as the last 'true' Habsburg. Her husband, Emperor Francis I, was of the House of Lorraine. The house resulting from their marriage was styled as the House of Habsburg-Lorraine, rather than simply remaining the House of Lorraine, and was often simply called the House of Habsburg. I'm not sure if this is what you were looking for, but it's the closest thing I could think of off the top of my head.