Adam Clarke on 1 Chronicles 16:39

Zadok the priest - Both Zadok and Abiathar were high priests at this time: the former David established at Gibeah, or Gibeon, where the ark had been all the days of Saul; and the latter he established at Jerusalem, where the ark now was: so there were two high priests, and two distinct services; but there was only one ark. How long the service at Gibeon was continued we cannot tell; the principal functions were no doubt performed at Jerusalem.