Unfortunately due to the way SLI works, the efficient method of capturing isn't available to it without it dropping half the frames of the capture, it's quite annoying.
You can do it with the multi-adapter compatibility option in the properties though, an option that wasn't in the original OBS. It will have a bit more of an impact than without that option though, because it's sort of a different capture method. If you do use it, I recommend using the scale option in game capture properties as well, and the "limit capture framerate" to reduce any impact as much as possible.
