Okay done, thanks to gizmo98, josy-cotter and foob.

I had bought 2 low-cost SNES USB controllers for PC/MAC (Tomee) and wanted to play 2-player NES games with them on my retropie distro.

Here is that part of /opt/retropie/configs/all/retroarch.cfg that configures both gamepads as well as sets up emulator exit via “start+select” buttons combo:

input_player1_joypad_index = 0
input_player2_joypad_index = 1

input_player1_b_btn = “2”
input_player1_y_btn = “3”
input_player1_select_btn = “8”
input_player1_start_btn = “9”
input_player1_up_axis = “-1”
input_player1_down_axis = “+1”
input_player1_left_axis = “-0”
input_player1_right_axis = “+0”
input_player1_a_btn = “1”
input_player1_x_btn = “0”
input_player1_l_btn = “4”
input_player1_r_btn = “5”
input_player2_b_btn = “2”
input_player2_y_btn = “3”
input_player2_select_btn = “8”
input_player2_start_btn = “9”
input_player2_up_axis = “-1”
input_player2_down_axis = “+1”
input_player2_left_axis = “-0”
input_player2_right_axis = “+0”
input_player2_a_btn = “1”
input_player2_x_btn = “0”
input_player2_l_btn = “4”
input_player2_r_btn = “5”

input_enable_hotkey_btn = “8”
input_exit_emulator_btn = “9”

I hope it helps.
Alex