The above is simpler and probably less frustrating. Alternatively you could create a global variable such as GV:"Player". Have it default set to '0'.
GV:Player = 0 (Controls Player 1)
When Player presses 'C', add '1' to GV:Player
If GV:Player = 1 (Controls/Playerbox deactivated, PlayerBox set to Player 2 position).
Then adds '1' to GV:Player
GV:Player = 2 (Controls Player 2)
When Player presses 'C', add '1' to GV:Player
So if GV:Player = 3 (Controls/Playerbox deactivated, PlayerBox set to Player 1 position).
Then adds '1' to GV:Player
GV:Player = 4 (When GV:Player =< 4, set GV:Player = 0)
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This means to change who the Playerbox controls you only need to set the value of GV:Player box to 1 or 3 and it should automatically re-position the Playerbox as the values change to 2 or 4(Acutally 0). This has an advantage if you are wanting to put transition animations in between the player swtich. But a disadvantage if all you want is a straight switch like mentioned in a comment above.