[sciolistse]

This is generally handled by an input cleaner, either in the controller itself if you hook it up to an original system, or via an emulator if you're using one. In the case of SMB1 L+R is disallowed (and it does give some in game time advantages) and all allowed emulators will give 'neutral' inputs if you press both L and R simultaneously so it will act as if you've pressed neither.

[stormbrew]

On the NES or SNES and any console with a dpad like those (which was patented by nintendo so until that expired there were other designed), it's actually just physically impossible. The directions are on a physical rocker that can't be pressed in two parallel directions at once.

Though if the controller pcb is really worn down by the rocker's center nub, it can start to become possible, and then there really is nothing stopping you. It's not uncommon for really well-loved controllers to end up like this.

It's definitely a letter-of-the-law thing where you could claim "but it's an original controller!" but no speedrun leaderboard is gonna let you get away with it.

Also I don't think emulators do a neutral input if you press both, that would be quite weird (and unlike how it happens with a real controller when you move from one to the other). Instead I think they prioritize the first input.

[sciolistse]

Yeah on original NES/SNES controllers the rocker physically prevented this. I was mostly referring to some third party controllers like say the 'hitbox' which go through an input cleaner before sending it to the system.

The way emulators handle L+R depends a bit.. For SMB1 specifically you're allowed to use an up to date version of Nestopia, Bizhawk, FCEUX or OpenEMU, all of which by default give neutral inputs if you press L+R (I don't recall if OpenEMU works this way but I believe it does.) There are other emulators like older versions of Bizhawk which gave priority to your latest input, or older versions of Mesen which gave priority to left and down. I'm sure other methods of handling L+R exist as well.

Which method is most correct is debateable, and sometimes contentious! :)