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
Thanks for the reply, I guess I will try it and see what happens. I will upgrade my PSU as well, that thing is 4 years old anyway..