Only 1 guitar can operate on a single dongle. Any additional guitars must use their own dongle that's compatable for the console that it's being used on.
No. The PS4 and Xbox One dongles are different in color and aren't compatable on other consoles except for the console(s) that are mentioned on the dongles themselves. It has to be the same dongle for the same console (ie. PS4 for PS4 console, Xbox One for Xbox One Console, etc.)