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As of today there remain a few control mapping use cases which are not addressed by the mapping system. This is a feature request to support those use cases.

"N to 1" / Multiple Gamepads to One RetroPad
Arcade games such as Robotron: 2084 feature a "Dual Joystick" control scheme where one joystick is held in each of the player's hands. In order to have a general purpose emulation system (where the two joysticks can usually be assigned to different players) that also fully supports Robotron, it is necessary to be able to map both joysticks to Player 1 on a game-by-game basis.

N64 games like Perfect Dark and Goldeneye would also benefit from this option by allowing the user to configure the Player 1 joystick for movement and Player 2 for direction for full analog movement. This is supported by some standalone emulators.

"1 to N" / One Gamepad to Multiple RetroPads
Some console games had additional features or cheats that are available for Player 1 if the player uses controls on the Player 2 gamepad of the original system -- for example, Megaman 3 on NES.

Other games, like Smash TV, require two controllers in their original versions because they are ports of "Dual Joystick" arcade games. Users with gamepads that include a joystick and a d-pad may wish to map both of the original NES d-pads to the same physical controller today.