There have been studies done, and referenced on this site as well as reef central that this is something of a myth and that these fish will be just as stressed out in your 6 footer as in anything viable for the average hobbyist. 
 
I'm contrarian. 
i think that 55 will be more than fine, especially for just 2 months. 
 
If it were me, Id be more worried about aggression between the two than anything and might just keep them in seperate, smaller tanks (maybe 20-30 gallons like you'd find these fish in at ANY LFS) until your big tank is ready. 
		
	
		
		
		
		
		
	
		
		
	
	
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