On the topic of making conditions on Eiruvei Techumin, Eruvin 3:5 rules:
אִם בָּא חָכָם מִן הַמִּזְרָח, עֵרוּבִי לַמִּזְרָח. מִן הַמַּעֲרָב, עֵרוּבִי לַמַּעֲרָב. בָּא לְכָאן וּלְכָאן, לִמְקוֹם שֶׁאֶרְצֶה אֵלֵךְ. לֹא לְכָאן וְלֹא לְכָאן, הֲרֵינִי כִבְנֵי עִירִי. רַבִּי יְהוּדָה אוֹמֵר, אִם הָיָה אֶחָד מֵהֶן רַבּוֹ, הוֹלֵךְ אֵצֶל רַבּוֹ, וְאִם הָיוּ שְׁנֵיהֶם רַבּוֹתָיו, לִמְקוֹם שֶׁיִּרְצֶה יֵלֵךְ:
[If a person makes a condition that] if a Chacham comes from the east, my Eruv should be [the one I set up] to the east; if he comes from the west, my Eruv should be to the west. If [a Chacham] comes from both directions, [my Eruv should be] whichever I want. If he doesn’t come from either direction, I’ll be like the people of my town [that is, no Eruv is valid]. R’ Yehuda says [regarding the case where a Rav comes from both sides], if one of them is his Rav, he goes to his Rav. If both are his Rav, he goes wherever he wants to go.
The Tiferes Yisrael notes that in the latter case of R’ Yehuda:
אפילו אחד מהן רבו מובהק
Even if one of them is his primary Rav
he can still go to whichever one he wants.
Why should this be? Given that he’s presumably going to learn Torah, clearly R’ Yehuda paskens that if exactly one of them is his Rav, he goes to him because he’s going to learn Torah from his Rav - that’s who he learns Torah from. If one of them is his primary Rav, of course he would go to learn from him over the other guy, following the same logic! So why is the Eruv left up in the air, since we should be able to assume which one he would want ahead of time, just as in the first case?