Depends on the render workload created by the resolution and detail settings, and it also depends on the specific games you play and the conditions you play them in.
The FX-6300 will have no problems keeping a pair of GTX760's busy in almost any game in almost any condition if they are driving a 4K monitor at ultra settings. No CPU bottleneck there.
On the other hand, if you're playing at 1080P and like to play compute intensive multiplayer games, chances are you are already experiencing CPU bound conditions. Adding another GPU will not improve your FPS minimums caused by the CPU in these types of games/conditions. The only reason to add another GTX760 in this case would be if you were planning to move up to a 1440P monitor (which...