Check the amperage on the 12 volt rail
If you have a single rail with more than about 38 amps it should be possible
If you have2 12 volt rails of 18 amps each one of them running the two graphics cards is going to get hot quite quickly
Try it and see . If the PSU is any good when it gets hot it may just shut down , rather than catch fire and burn your house down
it does have 1 rail, i knew this when I got it, hense the reason I spent the extra £15 on a decent brand.