MAME *has* support for dual screen games, this cabinet is just one of those lazy jobs showing the same image on both screens.
Run n Gun is a bit of an issue, because Konami especially were a pain with their dual screen setups (they actually use a custom chip paired with some kind of filter board to split the signal between two monitors each running the game at 30fps instead of 60, and the way it's hooked up isn't always obvious) Run and Gun is an especially odd case because it seems to be able to detect the filter board, and can run a single screen at 60fps instead. The sequel just has some issues with the rendering code (wrong sprites are sent to the 2nd monitor)
but there is plenty of Taito stuff, the Psikyo Mahjongs (+ Lode Runner Dig Fight), Sega Multi32 (Outrunners) and plenty of others which work just fine outputting 2+ screens in MAME, and if you have 2 monitors you can use them both (-numscreens 2)
2 screen version of darius.. if you use -window you get 2 windows instead of 2 screens.. omit -window if you have 2 monitors, and it will use both.
mame darius2d -numscreens 2 -window
or even 3 screens
mame darius2 -numscreens 3 -window