Gregory Paul has been running with that for a while now. T. rex is so similar to Tarbosaurus bataar, the Mongolian species might just as well be called Tyrannosaurus bataar.
I'm pretty sure Tarbosaurus is more distant relative that T Rex, the landbridge millions of years ago was open before T Rex and Tarbosaurus evolved, they have a common ancestor, it's like saying a oragatan and a gorilla are the same due to their skeletons looking similar
see, now we know they're birds because scientists are going to start nitpicking totally indistinguishable differences and calling them different species
I support the separation
*ducks from angry paleontologists
...although the species name from "Nanotyranus" should render one of them a junior synonym...
*ducks even further