While I thought they both would be happier together, I didn't expect to be able to tell a difference in their behavior, but I was in for a surprise! It took me 15 minutes of catch the bird that had been in quarantine and it was quite fearful; so I left the aviary immediately after releasing her. It took about 15 seconds for me to get out of the aviary, and when I turned to look, the two goldfinches were already together!
Since then, they both are more active, inquisitive and bold than before. They also chirp more (no real singing, I think I may have two females) and eat more. When alone, they almost never went below the top of their enclosures, but do now more often. It looks to me like when one is in an area where she feels less "safe" the other one stays back as lookout. I don't know but that's what it looks like to me.
Anyway, I couldn't be happier with the effect that being together has had on both of them.
