Damon and Pythias
Damon and Pythias were legendary Greek figures from Syracuse whose friendship symbolized deep loyalty to one another. When Pythias was condemned to death by Dionysius the Elder, he was released to make arrangements for his wife and children in preparation for his death, only because Damon stayed in his place and was ready to die if Pythias never returned. Some time later Pythias did return, and amazed by this act of loyalty, Dionysius the Elder freed them both. The countess uses this phrase in reference to how Mr. Crosbie must have stayed as long as he did at Allington because of his supposedly strong friendship with Bernard Dale. The countess could be using this Classical reference in a slightly mocking sense. [AM 2006]
Trollope himself had referred to Damon and Pythias in Chapter 2. [RR 2011]
Sources: Link to an English version of Valerius Maximus’ account of Damon and Pythias.