Make sure you both have ( the same version ) of T4m, and both have their texture settings set to 'manual'. And then 'normal' or 'high' should work.
There's some problems with t4m-coop for some people i believe. You could also try another making the other person host, and see if that works.

You say it is coop, but how do you create the lobby with the map to invite your friends. We both have the map working correctly but i dont know how to form the lobby for the map.