Thane is in general pretty mild, but with Shepard's presence he's starting to get a little bit more serious about trying to beat others so if they ran into each other while trying to get food or water or something he'd likely be willing to fight him.

alternatively, if i can get him injured before he meets up with Dualscar he'd probably fight him just to try and get some kind of cleaner death than bleeding or dehydrating to death and more attention.