Haha, it's a challenge. For me, I'm really fortunate to shoot my stuff on a white background, which means I can set the composite mode for one of the clips to "darken". Then I don't need to do any masking.
Other options are green screening, doing a "split screen" along some vertical edge that will hide the mask (like a door frame), or manually setting a mask frame-by-frame, which is probably the most time-intensive option.
If you have a solid background, doing a split-screen with some feathering can be nice and easy - just keep an eye out for how your lighting is set up. Shadows can ruin things.