Just find a quick workaround for retroarch. I added those lines to my config file :
input_player1_joypad_index = “0”
input_player2_joypad_index = “2”
input_player3_joypad_index = “3”
input_player4_joypad_index = “4”
input_player5_joypad_index = “1”
So controlblock 2 is always player 5, letting the space for usb gamepads.