ANSWER:
What they are doing is wrong and against the sunnah!
These 2 Imams are repressing the imam of the masjid and not themselves. Therefore, when one of them offers witr, and the other one offers another witr as the end of tahajjud, this is as if the Imam offered two witr in the same night, disobeying the Prophet salla Allahu Alaihi wa sallam.
For you, add another Rakah with the witr of taraweeh and then offer it at the end of tahajjud so that you would have only prayed one witr.