Carla, ‘étiez’ is the correct conjugation for 2nd person plural -  the sentence subject is ‘Emile et toi’ meaning the speaker is talking directly to at least one of the 2 people mentioned. The subject could be replaced by ‘vous’, the subject pronoun for 2nd person plural, and the verb conjugation accords with this. 

For emphasis, it could also have been written, ‘Emile et toi, vous étiez toujours poli’.