it was quit easy: make your room, select everythign except the floor, then copy it
rotate it on x axis 180 degree(ctrl-m)
then pres ctrl-i
and then put the copyed room underneat the other one,
make sure your floor is one unit thick and and a func_wall wiht a render mode of texture between 100-150 fx_amount
and watch out wiht the floor, you might make leaks