When the difference between actions is small or otherwise hard to compare side by side, overlaying the videos in the Analyzer's blend mode is the solution. The following video also demonstrates other video display controls; Flip, Zoom, Move, but of course these tools are also useful for single and split screen display modes.

Your questions answered

Is this how they do it when I see overlaid performances on TV?

No. Dartfish's Simulcam technology is used by many broadcasters (and won an Emmy award!); it creates a more realistic looking blend by identifying the pixels of the backgrounds so that they move together as the camera pans or zooms. Simulcam and Stromotion modules are available in the Pro editions of Dartfish.