Hamp's right, Hickok is probably the only one on record. In Wyatt Earp, Frontier Marshal, Earp mentions the carrying of two pistols more as a threat, of "having one in reserve", but doesn't mention using them both at the same time as a SASS Gunfighter.
Though flawed, some of the new Earp historians are giving more credit to much of the book.
Of course, I'd bet some of them Missouri boys shot two at a time a time or three, considering the amount of pistols credited to them carrying during the conflict....though there are no written accounts I'm aware of on it.
I think the traditional two gun fighter is probably more due to Hollywood than history. Once cartridge revolvers became more available, the carrying of two was a bit much for the average person.
Though a smaller hideout could easily have been hidden in a pocket or under a coat or vest. This was probably more common than the two full sized six shooters straped low on the hips.