I think that if a talent like this was added then a whole new class of weapon would have to be added as, as everyone has rightly pointed out, you can't DW bows or x-bows. You simply can't. End of story.
Having said that, they could easily replace the throwing stars / darts that rogues / warriors use and implement pistols instead which, in turn, could be DW'd by hunters.
Also, this wouldn't be in the MM tree because a marksman does not run around with 2 weapons out at once. Kinda defeats the point of being a 'marksman'...
Anyway, DW will never happen. Ever. It is a lame idea and doesn't fit in with the idea of a hunter. At all.