Can you fully list the system specs of a system that will not work. Please include make and model of PSU.

I would reinstall nvidia drivers. Remove current ones in Safe Mode using DriverFusion (free verison is fine) then add the cards and install drivers.
 
Have you tested if:
 
each card works separately;
2nd PCI-E slot works? you can check this by simply putting a single card into 2nd slot.
 
Might be that you need motherboard\s BIoS update

:
any new ideas anyone? Here is a complete list of my current system.
 
Intel Core i5-3570K 3.4GHz Quad-Core Processor (no OC)
Corsair H80i 77.0 CFM Liquid CPU Cooler
ASRock Z77 Extreme3 ATX LGA1155 Motherboard
G.Skill AEGIS 16GB (2 x 8GB) DDR3-1600 Memory
Samsung 840 EVO 500GB 2.5" Solid State Drive
Seagate Barracuda 3TB 3.5" 7200RPM Internal Hard Drive
EVGA GeForce GTX 760 2GB Dual Superclocked ACX Video Card(2-Way SLI)
EVGA GeForce GTX 760 2GB Dual Superclocked ACX Video Card
Cooler Master CM 690 II (Black) ATX Mid Tower Case
Thermaltake Smart M 850W 80+ Bronze Certified Semi-Modular ATX Power Supply
LG BH16NS40 Blu-Ray/DVD/CD Writer
Microsoft Windows 7 Ultimate Full 64-bit
Asus VH236H 23.0" Monitor
Asus VH236H 23.0" Monitor
Asus VH236H 23.0" Monitor
Creative Labs Sound Blaster Zx 24-bit 192 KHz Sound Card
Aerocool Touch 2000 Fan Controller
Logitech G15 Wired Standard Keyboard
Logitech G700s Wireless Laser Mouse
Astro A40 7.1 Channel Headset
Logitech G930 7.1 Channel Headset
 
the part numbers for the other 760s:  P/n:04G-P4-2768-KR