Some flight sim pilots may typically fly with two joysticks, and in some cases they might even be identical sticks. Very few devices provide any kind of unique ID to differentiate themselves from other devices of the same make & model, which makes it hard for X-Plane to differentiate between them in turn.
Starting in X-Plane 11.10, if you leave two identical devices constantly plugged in to their respective USB ports (or even unplug them but put them back into the exact same ports), the sim will do its best to provide a perfectly reasonable interface and experience—the devices should maintain their assignments across reboots, and generally “just work.”
But, if you unplug just one of them, or if you plug them into different ports, all bets are off. There are theoretical difficulties that make this an unsolvable problem, since literally the only thing X-Plane has to go on for telling them apart is which USB port they’re plugged in to.