It's a particularly good example because of how great they both are. One could easily cite terrible rap groups as they may be evenly as bad as each other (ICP, etc). But these guys were both of the same level in '98. I also agree that Mos is ahead of Kweli, at least on the level of being much more interesting a rapper, as well as progressive.
Only other good example that comes to mind would be Nas + AZ, but that doesn't feel like a true duo in the sense of what we're looking for here.