When you try to mix arbitrary video sources, there is the problem that one image is for example on the top left and the second image is in the middle; both images are therefore not synchronized. In the past and still in some professional equipment, this is solved by synchronizing the video sources with a pulse coming from the mixer.
However, with today’s digital cameras and cheap digital equipment, this is no longer necessary at all. Many AV mixers have a so-called frame store in which a frame of the camera is always stored and mixed with it. This way you can mix multiple cameras, DV player, DVD player and other sources.
The disadvantage of this method is that the output of the image mixer is a few frames behind the input. So if the sound wants to stay in sync, the sound must be delayed the same number of frames. Many AV mixers have this capability built in.
There are several brands of video mixers: Datavideo and Blackmagic are examples of brands that have affordable solutions. SMTS also always conducts research into completely software solutions (PC or MAC with software).