One of the things I assumed earlier on was that I would want to use SLI, however this is not true. SLI is useful for games when you want multiple graphics cards to look like a single card. But in the case of using Tensor Flow or cuDNN, one just needs to specify which GPU device you want each part of the code to run on, and there's no need to pretend each one is a single device. So I didn't bother buying any SLI bridges or whatever.