If I could only have 2 chickens I would for sure want them to look different for variety. They are going to be best buds regardless of breed.
You would hopefully think two chickens should be best buds. We had a Buff Leghorn and Blue Wheaten Ameraucana very similar in size and color. However, the Buff Leg took advantage of the Ameraucana's non-combative gentle nature and lorded over her, chasing her, pulling out her muff/beard down to the skin, and the Amer would not fight back. Best friends? Hmmm - I don't think so. Not only should both hens be similar in size, but also should have similar temperaments. Example: I would never put a docile Faverolles with an aggressive heavy dual purpose breed like a RIR, BR, Orp, 'Lorp, Wyandotte, etc. The Faverolles would never stand up for itself in a challenge the way our Amer didn't.