Take an entire hardware-based TV editing studio and put the controls on a GUI. The version I played with (in the early 90s, and it was already a couple of years old then) had four coax inputs, two coax outputs and a vga output. The coax connections also carried control information to start/stop/record/etc the 1-inch tape decks they connected to. The vga was for the monitor that had the GUI.
You could have four independent video feeds, mix them down in the GUI -- which had a real-time thumbnail view of your output -- and two independent video outputs. All this from a small form factor box, probably comparable to an ATX case.
I remember hearing at the time one of the pieces of magic that made this possible was that the system clock was syncronized to the NTSC clock the video ran on. As long as you keep the system closk at an integer multiple of NTSC, real-time compositing becomes trivial.