The conjunction mientras can be used to join two verbs in the imperfect, or it can be used in sentences where one verb is in the preterite and the other is in the imperfect. The sentence will change meaning depending on the tense of the verbs joined by mientras.
ex of imperf, imperf: Yo cocinaba mientras mi hermana limpiaba.
ex of pret, imperf: Yo cociné la cena mientras mi hermana limpiaba.
