King Crimson is correct, the so-called "double trio" had two drummers (Bill Bruford and Pat Mastelloto) and two guitarists (Robert Fripp and Adrian Belew) as well as two "bassists"—Tony Levin and Trey Gunn. If you want to be a stickler they weren't really bassists. Tony Levin plays bass but on some number of KC tunes he played
the Chapman Stick, though often filling the bass role anyway. Trey Gunn technically plays something called the Warr touch guitar, which is a Chapman Stick-like instrument. While often in the bass role he also plays melodic and harmonic stuff too. And of course the material was carefully arranged to keep the parts distinct and out of each other's way.
Stanley Clarke had a group for a while where he had another bass player to fill the traditional bass role sometimes while he played up in the guitar's range on his so-called piccollo bass.
Speaking of Stanley, he was on a 1971 recording of Pharoah Sanders' called
Live at the East with a second bassist, Cecil McBee, though here we're talking uprights, not electric bass guitars. There's a fantastic passage where the two of them play an incredible improvised duet, one of the best things SC ever recorded, IMHO, though it's little-known. A couple of years later and Stanley got busy becoming the prototypical fusion electric bass monster, but his upright playing was always what I liked most from him.