You can't have 2 different graphics card, you must have 2 equal graphics cards with SLI support.
Having 2 GPUs is a huge help since you'd (in theory) split the usage between the GPUs.
In systems with 2 GPUs you got a much more consistent Fps since there are 2 GPUs running the game.
I said 'in theory' because this work split sometimes does not happen, a game must be optimized for multiple GPUs, some games doesn't need multiple GPUs to get the best graphics and FPS, so even though there are 2 graphics card, the game is going to use almost only one.
The same work for processors with multiple cores, if you got a a 8 core processor and play a game like league of legends, that can be perfectly played with only two, you're going to see 2 cores working much than the other's 6, because that's how the game was designed.
Before making a PC with 2 GPU take a look at the games that you're going to play and if they work well of 2 graphics card, although I wouldn't recommend that, since only one 1080 can crush almost any game on this gen.