Urriak Uruk
Gaming is fun, and fun is for everyone
I mean, it works however a given DM wants it to work. But for official D&D canon, they have chosen a Platonist approach to divine beings, while connecting the different worlds. Eberron characters and elements have even been appearing in official sources and streams, crossing the boundaries. Doesn't mean that has to be true of any given tables Eberron, or Krynn...but that's how WotC is choosing to do the formal canon.
Well, considering how Takhisis has been killed in Dragonlance's timeline, I don't think we can say Tiamat and Takhisis are EXACTLY the same. I find them are different repesentations of the same god, like a prism making different colors out of the same beam of light. But that's me I suppose.