Koolance introduced a series of video card water connecting blocks for the GeForce GTX 780 and GeForce GTX Titan graphics cards. There are four versions available, including one for 2 cards, a version for 2 cards with an empty slot in between them, one for 3 cards and one for 4 cards. You can find the details and specifications over here, pricing starts at $39.49.
Koolance video bridge connectors link multiple video water blocks together with ease. Flow can be directed in series or parallel.
NOTE: Hose barbs are recommended for 10mm (3/8in) and 13mm (1/2in) ID tubing sizes to avoid conflicting with the water block or video card.