There are many options available to connect multiple amplifiers to one source. These options vary from one system to another as different systems have different input/output options.
You may be connecting multiple amps for different purposes like if you have an old amplifier that you think could add value to your audio system.
Most of the people connect two amps together; one for speakers and other for subwoofers. It gives better sound quality.
Some people also attach multiple amps to hear the louder sound. Whatever the reason is, you can wire multiple amps in 4 different ways.