The current technique used for today's DX11 games is Alternate Frame Rendering (AFR). In practice if the game engine supports it you get very good scaling. Problem with is technique is that most game engines are developed for consoles and single GPU. They render one frame but share info with the other frame. Because info is reused/passed from one frame to another it's next to impossible to split them so one GPU does one and the other GPU does the other frame. You notice in some games you have to turn off AA because usually AA techniques are sharing information between frames. In the past there was a technique called. Split Frame Rendering (SFR).
In this method one card does the top half and the other card does the bottom half. I have never tested this personally. It's suppose to be the method we will be using with DX12 in the future.