Well, when I went outside for the first time this morning I saw both baby birds, one clearly a lot bigger, in the nest. But looking at the two of them more closely, in better light, made me think that the older one might have been the one I saw yesterday afternoon, with the younger one concealed from view. Even the older one didn't look flight-ready yet. Then, when I checked on the nest a little later, it had gotten a lot more crowded:
When I stepped back to get a different angle, I noticed that the parent and two offspring in the nest were also being watched over by the other parent.
I'm not sure how to tell which one of the grown-ups was the male and which one the female, but they are clearly an effective mutual aid society. I couldn't stay outside too long because the bird on the roof started to get very agitated. I didn't want it to have to resort to the my-wing-is-broken-routine, so I came inside.