If you mean the one cable from your psu...I don't think that's a good idea 'cos that would be using the one rail. You would then be pushing twice the amps through that one cable on the psu.
I was thinking much the same, however, the manual says the PSU can power 2 GFX cards. What's the point having two plugs per cable if you can't use them both simultaneously.
As there are only 2 cables, each providing 2 plugs, if I want to go SLI then I will have to power each card with one cable.
Hmmmmmmm... still not sure!