There are numerous synonyms for the phrase "were acquainted" that you can use to describe two people who know each other. Some possible options include "were familiar," "knew each other," "had met," "were introduced," "had crossed paths," "had been in contact," "had exchanged greetings," "were on speaking terms," "were well acquainted," and "were old acquaintances." Each of these phrases can be used to convey different shades of meaning depending on the context, such as whether the acquaintanceship was casual or long-standing, friendly or formal, or based on direct interactions or third-party connections.