Here's what I would do, unlock the phase 1 doors, but keep the doors leading to phase 3 locked. Keep everything else the same. That way, a single player and go to the various areas of phase 1 and 2 at will, but they will still be required to complete both to move on. That's what I did when I was testing way back when I wrote that time sink pain in the ass zone. 
		
	
		
		
		
		
		
		
		
		
		
	
		
		
	
	
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