I think 2 would just cause problems in the pecking order unless they are in separate areas of the yard. You could try putting more chic,ens in there but do not exceed 16 birds.
As long as you are letting your chickens out every morning really early it should be okay.
Let me just say, I do not agree with the space Justin Rhodes gives his hens. A full grown hen needs a minimum of 4 sq feet of coop space. So for 20 chickens the coop would have to be as small as 10x8 feet