It's not clear whether private homes even have more than one house-elf: Lucius' "You lost me my servant", Draco's "He dresses like our old house elf" (both singular) and Kreacher's presence as the only House Elf in the Black home suggest that Dobby was the only elf at the Malfoy's, which again brings your question to mind:

Who in the Malfoy household could possibly have told him Dumbledore was a great Headmaster?