Maybe on this link you can find something useful.
https://forums.bohemia.net/forums/topic/194077-xbox-controller-sensitivity-issue/
or this helps:
"Fixed: Multiple analog inputs on one action are averaged instead of summed. Be sure to not set input you are not using for that action as this will lower the overall input."
This was fixed along time ago,i hadnt played with multiple bindings until recently. Despite the other joystick not being connected the games decides to use the average. 0 for the disconnected/disabled joystick and 100 for the connected gives 50%. This should apply to any other analog input like a gamepad. It wont work if you just disable the analog input, you need to remove the bind.
I hope the devs look into this because you should be able to add multiple inputs, maybe not at the same time but it shouldnt count controllers thats not connected or disabled...