I agree with Danbuscus. If you can afford two Titan Black's I would simply recommend getting one Titan X and then waiting to get another a few months down the road.
SLI is finicky, and it greatly depends on what you're using it for.
A rule of thumb is to, instead, buy the single fastest GPU you can afford, rather than two weaker ones.
Unless you can get the two weaker ones for a ridiculously low price, I suppose.