Please forgive me if I misunderstood your meaning......and here you go:
Ctrl-# (where # is from 1-4) Toggles AI for the #th player
So, you will need to press keyboard key Ctrl-2 when you are in VS modes...........
The AI toggle effect lasts for the whole match during VS mode, Team VS (Single & Simul), but not for match during Team VS (Turns).
From MUGEN readme:
IV. Game HotKeys
=====================================================================
These are the hotkeys recognized at the fight screen:
Key Function
--- --------
Pause Toggle pause
ScrollLck Frame-step within pause
Esc Quit
The following hotkeys are for debugging purposes and can be disabled
by setting AllowDebugKeys = 0 in mugen.cfg.
Key Function
--- --------
F1 Sets Player 2's life to zero
Ctrl-F1 Sets Player 1's life to zero
F2 Sets both players' life to 1
Ctrl-F2 Sets Player 1's life to 1
Shift-F2 Sets Player 2's life to 1
F3 Gives both players full power
F4 Reset the round
Shift-F4 Reloads stage, characters and fight data
F5 Time Over
F12 Take a screenshot (saved to mugen?.pcx)
Ctrl-C Toggles display of collision boxes, target data (including
remaining juggle points) and NotHitBy attributes
Ctrl-D Toggles debug information display
Ctrl-I Forces both players into stand state
Ctrl-L Toggles display of the life and power bars
Ctrl-S Run the game as fast as possible
Ctrl-V Enable V-sync (stops "shearing")
Ctrl-# (where # is from 1-4) Toggles AI for the #th player
Ctrl-Alt-# (where # is from 1-4) Enables/Disables the player
Space Restores full life and power to all players