If someone keeps Rabbeinu Tam’s zeman for Motzo’ei Shabbos, may he ask his friend who already ‘finished’ Shabbos to do melacha for him?
That question is delat with by Rav Shmuel Halevi Wosner, the well known senior posek and rosh yeshiva of Yeshiva Chachmei Lublin in Bnei Brak. In Shevet Halevi (1:53), he states that you may not ask the other Yid to do this. He says that this may apply even if you only keep Rabbeinu Tam on Motzo’ei Shabbos a a chumrah, but at all other times you go by the earlier zeman.
Rav Moshe Shternbuch, Rosh Av Bais Din of the Eidah Hachareidis, in Teshuvos Vehanhagos (1:234) rules that one may do so if he only holds Rabbeinu Tam for things that are min HaTorah and not DeRabbonons.