To give us some insight on what to expect from DX12's implementation of multi-GPU, AMD's Scott Wasson, Senior Manager, Product Marketing at AMD, offers a video walk-through of the technology, putting it in simple terms we can all understand. It's well worth a watch if you are able to afford five minutes of aural and visual distraction.
Frame pacing comes to DirectX 12 for AMD through a Radeon driver update that allows alternate frame-rendering to work without causing stuttering in the game engine, even in the event that one frame requires more processing power than another. "Frame pacing" is simply a way for the GPUs to intelligently understand which GPU should produce which frame so that the end-result is as smooth as possible an animation for the gamer. While the hardware has to play a role in this as well, AMD has proven with its latest driver release that the software is at least half the brain.
An example of smart frame pacing can be seen below. Ordinarily, one frame to the next could be wildly different in terms of render times, especially in grueling parts of a game. What you see below is a result of good frame pacing, where each frame is almost perfectly uniform - perfect enough to where a normal person would never notice an issue. Most gamers could easily notice an issue without frame pacing, however.