Maksim Chmerkovskiy & Peta Murgatroyd -- Dancing with Their Tongues

"Dancing with the Stars" rump shakers Maksim Chmerkovskiy and Peta Murgatroyd are real pros ... at sucking each other's faces.

The rumored couple hit up XS nightclub in Las Vegas this past weekend and got caught doing a very passionate lip tango. Tens across the board.

Looks like Peta's real prize for winning the DWTS championship with NFL star Donald Driver last week wasn't a mirror ball trophy after all -- it was Maksim.