I have come across a couple of individuals of Quora who have made the claim that the Crimean Khans were considered a reserve dynasty by the Ottomans who would rule if the Ottoman dynasty ceased to exist. Was this actually so? If so was it actually a legal thing or just an idea that was suggested? Was a reserve dynasty a thing that any other states had?
I have never heard of this and I study the Classical Era of the Ottoman Empire. If it is true I could maybe see it coming from a text of "mirror for princes" type literature (basically political advice texts)...but even then this is a stretch. The Ottoman state was the House of Osman both practically and ideologically. Even the military revolts of the late sixteenth and seventeenth centuries believed that they were prolonging the state in some way. If this claim is true I would like to see a source because it is an interesting concept. I wish I could provide a more substantive answer but I have never come across this concept in any Ottoman studies literature that I have read.
The Crimean Tatars were a protectorate and military auxiliary of the empire, but the primary sources I have read never refer to the Tatar khan with the same title as the Ottoman emperor. For some perspective, the Habsburg Holy Roman Emperor was referred to as the King of the Germans until the early 1600s. If this "reserve dynasty" concept is the case I would think the Tatar khan would be given more dignity in Ottoman documents. Maybe one of the other Ottoman history people here can give more information on if this was true or where this idea came from at least.