In your example (for 2-5 players), player number 2 has control of joystick port 0 (in joystick mode, using keyboard as joystick emulation). Player two should then be able to effectively press LMB by pressing the fire button.
If it is important that also player 1 can do this, you could map a key on the keyboard for player 1 to action_joy_0_fire_button (which is also the mouse button).
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