Can a traveler pray 'Isha behind a congregation praying Maghrib, and how would he follow them?

If a traveler prays behind a resident, he must complete the prayer and is not permitted to shorten it, whether he joins the Imam from the beginning of the prayer or during its course. For instance, if a traveler prays Isha behind someone praying Maghrib, he must complete the prayer and not shorten it. However, it is preferable for him not to pray Isha behind someone praying Maghrib, to avoid scholarly disagreement.