What is the ruling on praying Witr with the first imam and then leaving, or completing the prayer with the second imam while delaying Witr, or not praying Witr with the first imam and waiting for the second imam to pray Witr with him, and is that considered standing with the imam until he departs?

If there are two imams in the mosque, the virtue of qiyam al-layl (night prayer) with the imam is not attained until he departs, except by staying with both imams until they finish the prayer, because the second imam complements the first. The worshipper can make the witr of the first imam even-numbered with one rak'ah (unit of prayer) to pray with the second imam and pray witr at the end of the night. There is no blame upon one who departs after the first imam, but the complete virtue is only attained by staying with both of them.