what the hell is mental ray cloth, there is no such thing.
Theres Maya cloth (Maya 4 - 8), Ncloth (8.5) and Syflex.

anyway, did you make the pants collidable ???.

If that does not work try something I have used before.

When using Sylfex I would use the cloth to drive a dupulate that is wrapped deformed and made that collidable, as sometimes cloth on cloth does not work.

other than that update to 8.5 as ncloth rules