How do you transmit 2 video signals down one cable?

Answer 1

Generally it isn't possible to send two video signals on a single wire although there are instances where it can happen.

Video over Ethernet allows multiple signals to be sent in packets although the nature of Ethernet means that the signals will each have a delay so synchronizing the signals may be difficult.

Video over CAT5 uses converters that take a video signal and modify it to run over a single pair of a CAT5 cable. Normally, two more pairs would be used for stereo audio. If mono audio is used, it is possible to run two composite video signals with audio down one cable. Note that video over CAT5 uses the same cable as Ethernet but employs a very different signal format to Ethernet.

Composite video signals use a single cable but in fact, the signal has three separate video components encoded into the signal. The signals are one for brightness and two for color information. Together they make up a full color signal and can be separated into the three components at the far end. Although the cable carries three signals, remember that they are all parts of the same color signal so this probably doesn't qualify as two signals.

Finally, cable broadcasters use encoding systems to allow numerous signals to be carried on a single cable. The one that enters a house to supply a cable receiver carries many signals with just one being decoded and output by the receiver. This takes complex hardware at both ends so is outside the scope of most home users.