The ART Pro Audio CoolSwitch A/B-Y Switching Pedal works two ways. It lets you switch a common source signal (guitar, bass or keyboard) between two amplifiers or amp channels. Or conversely, it lets you switch two source signals (ie, two different guitars) into the same amplifier.
A good quality A/B-Y switcher will also provide LEDs or other indicators to show whether A, B or Y routing is in effect. The Y routing combines both the A and B outputs in parallel so that two amplifiers can be driven at once. The CoolSWITCH provides full A/B-Y switching functionality, plus parallel Common in/out jacks.