I added more debug output, but I can't see anything obviously wrong:

dinput   D         wjoydxnu.cpp:827 joystick_enum_callback           [  15.21407] joystick_enum_callback
dinput   D         wjoydxnu.cpp:967 joystick_enum_callback           [  15.23810] waker_event: 6cc
dinput   I         wjoydxnu.cpp:1009 joystick_enum_callback           [  15.24244] Joystick 0 initialized, GUID: f8bd5190-fbd6-11e8-8014445535400
dinput   I         wjoydxnu.cpp:1144 joydx_merge                      [  15.25648] Merged, num joysticks=1

dinput   D         wjoydxnu.cpp:827 joystick_enum_callback           [  16.36973] joystick_enum_callback
dinput   D         wjoydxnu.cpp:891 joystick_enum_callback           [  16.37197] Device f8bd5190-fbd6-11e8-8014445535400 still exists

dinput   D         wjoydxnu.cpp:827 joystick_enum_callback           [  21.42322] joystick_enum_callback
dinput   D         wjoydxnu.cpp:891 joystick_enum_callback           [  21.42815] Device f8bd5190-fbd6-11e8-8014445535400 still exists
dinput   D         wjoydxnu.cpp:827 joystick_enum_callback           [  21.51066] joystick_enum_callback
dinput   D         wjoydxnu.cpp:967 joystick_enum_callback           [  21.51245] waker_event: 77c
dinput   I         wjoydxnu.cpp:1009 joystick_enum_callback           [  21.51254] Joystick 1 initialized, GUID: f8c25aa0-fbd6-11e8-8024445535400
dinput   I         wjoydxnu.cpp:1144 joydx_merge                      [  21.51383] Merged, num joysticks=2

dinput   D         wjoydxnu.cpp:827 joystick_enum_callback           [  22.57566] joystick_enum_callback
dinput   D         wjoydxnu.cpp:891 joystick_enum_callback           [  22.57682] Device f8bd5190-fbd6-11e8-8014445535400 still exists
dinput   D         wjoydxnu.cpp:827 joystick_enum_callback           [  22.64180] joystick_enum_callback
dinput   D         wjoydxnu.cpp:891 joystick_enum_callback           [  22.64213] Device f8c25aa0-fbd6-11e8-8024445535400 still exists

dinput   D         wjoydxnu.cpp:418 _al_win_joystick_dinput_unacquire [  24.42247] Unacquiring joystick device at slot 0
dinput   D         wjoydxnu.cpp:418 _al_win_joystick_dinput_unacquire [  24.42839] Unacquiring joystick device at slot 1

(PRESSED BUTTON ON ONE CONTROLLER - handle_button_event CALLED FOR BOTH)

dinput   D         wjoydxnu.cpp:1439 joydx_thread_proc                [  27.00212] joydx_thread_proc, waker_num = 1, waker handle = 77c
dinput   D         wjoydxnu.cpp:1443 joydx_thread_proc                [  27.00230] update 1
dinput   D         wjoydxnu.cpp:1472 update_joystick                  [  27.00237] update_joystick 3f0d2748
dinput   D         wjoydxnu.cpp:1624 handle_button_event              [  27.00245] handle_button_event 3f0d2748, 0
dinput   D         wjoydxnu.cpp:1439 joydx_thread_proc                [  27.00254] joydx_thread_proc, waker_num = 0, waker handle = 6cc
dinput   D         wjoydxnu.cpp:1443 joydx_thread_proc                [  27.00264] update 0
dinput   D         wjoydxnu.cpp:1472 update_joystick                  [  27.00279] update_joystick 3f0d1fa0
dinput   D         wjoydxnu.cpp:1624 handle_button_event              [  27.00286] handle_button_event 3f0d1fa0, 0

0: New player: 4, controls: Controller (XBOX 360 For Windows)
0: New player: 5, controls: Controller (XBOX 360 For Windows)

(RELEASE OF BUTTON)

dinput   D         wjoydxnu.cpp:1439 joydx_thread_proc                [  27.11002] joydx_thread_proc, waker_num = 1, waker handle = 77c
dinput   D         wjoydxnu.cpp:1443 joydx_thread_proc                [  27.11018] update 1
dinput   D         wjoydxnu.cpp:1472 update_joystick                  [  27.11026] update_joystick 3f0d2748
dinput   D         wjoydxnu.cpp:1624 handle_button_event              [  27.11035] handle_button_event 3f0d2748, 0
dinput   D         wjoydxnu.cpp:1439 joydx_thread_proc                [  27.11042] joydx_thread_proc, waker_num = 0, waker handle = 6cc
dinput   D         wjoydxnu.cpp:1443 joydx_thread_proc                [  27.11048] update 0
dinput   D         wjoydxnu.cpp:1472 update_joystick                  [  27.11054] update_joystick 3f0d1fa0
dinput   D         wjoydxnu.cpp:1624 handle_button_event              [  27.11062] handle_button_event 3f0d1fa0, 0