In the 4e cosmology, Bahamut and Tiamat were created when the brooding loner Io tried to take on a Primordial all by himself. For his trouble, Io got literally cleft down the middle into two halves...which almost instantly re-formed into the siblings. They then kicked the Primordial's s#!t (one of the few Primordials actually slain IIRC). So...all the evil, mean, and wicked parts of the God of Balance got split off into Tiamat, and the nice and noble parts got sent to Bahamut. Headcanon: Bahamut is the ultimate god of Good.

Also, separately: Bahamut and Kord are lovers. Their relationship can be tempestuous (figuratively and literally...) but they always patch it back up. That's why Kord hangs out in Celestia, despite it not being his own divine domain (since he seems to lack one).