There's only one other one and if I remove that the CPU gets no cool air at all. Plus I don't even think a second (well third, there's already one in the back for the CPU) rad would fit.
A Nano on the other hand would just drop in there without disturbing anything.
Pluses? well I would bet my rear end that two Nanos would compete with two Titan X in SLI given that two Fury X are quite far ahead. Crossfire (when it does actually work) scales far better than SLI. So even if the speed of the X drops it would still be beastly.. I have a feeling it would be Fury X2 sort of performance any way.
I have absolutely no doubt at all that some one out there is going to test it for me any way, so there wouldn't be any risk involved