The info for the Titan X states 600W are recommended for this single GPU and the whole system.
So if that’s the max. power consumption of a single Titan X + rest of the system, the additional 1080Ti might need 250W extra as stated on the other page.
Maybe you were lucky in TF as the GPUs might not have been at their peak power consumption.
You could try to set the power limits lower using nvidia-smi, maybe even create artificial bottlenecks so that your GPUs will only have a short burst of power consumption or test another PSU if available.