Odd Hummingbird Behavior

Yesterday afternoon and this morning I saw something I’ve never seen before: two hummingbirds seemingly co-guarding one feeder. The feeder in question dangles from a clothesline in our small “rose garden,” a patio that is covered with shade cloth this time of year.
I believe they are juvenile Costa’s. Both are often on the clothesline together. Sometimes they seem to take turns drinking from the feeder. Sometimes one will display at the other. Sometimes one will chase the other. Once I saw them “dueling” with their beaks.  But most of the time they just sit there, checking out each other and the general area. Sometimes they sit several inches apart, as in the above photo, and sometimes they sit right next to each other—once about half an inch apart.
Here is my theory: I think they are nestmates, and there is some faint memory that keeps them from inflicting total mayhem on one another. They are obviously uneasy with the situation, but do accept it.
Other hummers have tried to horn in, but these guys so far have chased everyone else away.