I suppose if you got one for free. You could get some small gains. Considering what they sell for. It would make more sense to run a single GeForce GTX 1070. As it would be faster and use a whole lot less power.
If you were looking to buy one. You could sell the one you had. Add the budget for the second one and buy a new GTX 1070. If you had both. Sell them both for the same outcome.