If the story takes place before the 1960s and definitely by the 1970s or later, then a two handed stance is common place. But ALL historical material shows shooters using one hand, standing sideways to present the slimmest target to their opponent, arm extended.
I had to argue with an art director once, who had and actor playing SHERLOCK HOLMES take the 'horse stance' two handed with his Webley MK1. It looked like a 70s cop show, except with a Victorian Gentleman. It looked really anachronistic, which it was!
