Classic Duelling Techniigues(not that it was legal mind you) tells us ,as to give the opponent as small as target as possible, hence standing sideways profile, with arm full extended(one handed) toward your opponent.....
and many of the Pistols were meant for Calvary, therefore a one handed grip would be necessary, other to be controling the horse.
even a quick search on google brings up many (from the 18th century) woodcuts of famous duells which show a one handed stance....
i'll dig a little deeper later looking thru some of my weapons and tactic books, looking for when the 2 handed stance was adopted