Is one 6 pin to 8 pin ( 6 + 2) enough

[PlanetBBQ]

I have an old Cooler Master silent pro 850w which has 6 x 6 pin pcie slots, i have two 6 pin male to 8 pin (6+ 2) leads only. I want to hook up 2 x GTX 780's i have kicking about. Will one lead per card be enough?. I would prefer to use two per card, but cannot find 6 pin male to 8 pin( 6+ 2) male online (in stock that is)