Are you overclocking the CPU at all? Your CPU uses more power than a 3770k already and total system usage with a single 760 is something like 350 watts already. If you add 200 for the next video card and an extra 40 for your CPU you's be looking at needing to supply 590 (or we will just round up to 600 watts). Generally you want to stay below 80% of your power supplies capacity, so even if it's rated at 600 watts total and can actually supply it properly at that amount for long periods of time I doubt it would be stable.
Granted I'm overestimating a bit based on just rough ideas of how much each part consumes. but that seems like it would be pushing the supply way to hard, even if it is an amazingly built supply.
Here is Guru3D's power supply recommendation:
•GeForce GTX 760 - On your average system the card requires you to have a 550 Watt power supply unit.
•GeForce GTX 760 2x SLI - On your average system the cards require you to have a 700~800 Watt power supply unit as minimum. http://www.guru3d.com/articles_pages/geforce_gtx_760_sl...