Who Know and there are different answers.

Allman Bros had two because Butch and Jaimo had such different styles that blended so well together and having the two of them contributed to their unique sound (now there are probably two because Butch is over 60!! - sorry Butch).

I was in a band in the late 60s and we used two drummers because one of them spent at least 1/2 of his time as a frontman singer.

You can punish yourself and see the reunion of 8 old men with two drummers at youtube searching at music for mouseandtheboys (I am not recommending this) and see one old guy banging on traditional drums and the other old guy banging on electronic drum kit. I am the only one with enough hair for a ponytail.