The current behaviour is for only one input to be able to control a given axis, but as raised here there are benefits to supporting multiple inputs assigned to the same axis, for various advanced control scenarios.

In particular,

with "conflict" resolution handled by just summing up the inputs from all inputs mapped to a given axis:

• as Willian has previously suggested this would allow using the triggers as up/down inputs
• it would also allow using something like a slider to set a general level for an axis, then a spring-loaded joystick to handle finer-grain control around that set point
• it could be used as a power limiter, whereby joystick 1 can only do up to 50% of the possible axis range, and consciously also using joystick 2 allows "overloading" to use higher levels, which some controllers might want to use as a safety mechanism (in the same way that you need two keys to unlock some doors, like if the vehicle could restart itself if you overload without knowing what you're doing)
• it could also set things up nicely for multi-joystick control, with a trainer and student, especially if we could set some kind of numerical weighting to the values from each input (rather than requiring one to be a strict override of the other)