It's like mixing Coke and Pepsi.
A brave, perhaps slightly mad modder is working on bringing as much of League of Legends as he can to Valve's DOTA 2, from LoL's champions to its creeps.
Those champions are being imported "1:1", meaning its not just the 3D models being brought over (though these are being reworked since the importing process roughs them up a little), but their traits and specs as well.
Which means, yes, you can pit LoL champions against DOTA 2's heroes.
You can follow the project's progress here.