Running one ASUS HD 7770 DC 1GD5, adding a second makes screens blank

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Guest

Guest
On one if my machines, I've been running 3 monitors on an ASUS HD7770 GPU. I recently added a second, and the result has been blank (although lit) screens. I'm currently running on a ASUS P7P55D-E LGA 1156 Intel P55 SATA 6Gb/s MB, with a 650W Total PSU. Any ideas as to what I've done wrong, or if I accidentally skipped a step?
 
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Guest

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I have the cards physically bridged, but I'm not sure if there is a setting or anything to actually enable crossfire.