not being by the same company should have no effect at all.
my little brother and I have both had sli setups in the past with different card vendors and have never had a problem(due to the cards being different manufacturers that is there are always issues with sli in my experience)
and Im not sure on if your psu would cope... as a 260 can pull 185 watts by itself... meaning you would need about 380 spare watts for your gpu's soo it depends on the rest of your system