I was the second drummer in a two-drummer band for a while! It was really fun, and definitely made a fatter sound.
What amazed me was how easy it was to sync up. Neither of us were really great players, but to my ears we were very rarely flamming or very out of step. I've thought about this a lot and I really think it just naturally worked well for us, and I think it would for any two drummers with halfway decent ears.
That said, it was not really necessary, and wayyy more of a pain in the throne than it was worth in this particular band (thankfully we had roadies, as there was also a b3 player with a leslie!).
Stereolab and Broken Social Scene both use multiple drummers on some recordings to great effect, I think. They end up with a specific kind of "big" sound that seems programmed because there's so much going on, like more polyphany than a single drummer could produce, if that makes sense.
With that in mind, I've done multiple drum takes when recording and layered them for that kind of effect and really dig it. It just isn't feasible for my bands to have multiple drummers!
Looking forward to other people's take on multiple drummer bands!