This revollver doesn't make sense. There WAS a double barreled revolver ( https://youtu.be/mqdQrpF2PmI ), but it had two concentric rows of chambers in the cylinder. This gun fires two shots at once reducing a 6-shot gun to a 3-shot gun.
There is a two barreled volley revolver ( https://en.wikipedia.org/wiki/S333_Thunderstruck ) with the trigger characteristics of a staple gun, but that gun has its two barrels side by side to simplify the striker firing mechanism. This gun obviously fires the two chambers on opposing sides of the cylinder at once. I have no idea, how the the firing mechanism should work, because it's not striker fired.

sanitaeter said in comment #2613061:

This revollver doesn't make sense. There WAS a double barreled revolver ( https://youtu.be/mqdQrpF2PmI ), but it had two concentric rows of chambers in the cylinder. This gun fires two shots at once reducing a 6-shot gun to a 3-shot gun.
There is a two barreled volley revolver ( https://en.wikipedia.org/wiki/S333_Thunderstruck ) with the trigger characteristics of a staple gun, but that gun has its two barrels side by side to simplify the striker firing mechanism. This gun obviously fires the two chambers on opposing sides of the cylinder at once. I have no idea, how the the firing mechanism should work, because it's not striker fired.

I'm pretty sure this is Nero's revolver from DMC 4 since the artist's commentary mention DMC