Do Not Be Inconsistent
12. Establish a minhag. Since the halacha is not determined conclusively like the Chachamim or Rebbi Yehuda, one may choose to follow the Chachamim or Rebbi Yehuda, provided that he sticks with one way, l’chatchila always (שו''ע סי' רל''ג ס''א ). If he follows Rebbi Yehuda and davens Maariv starting at plag, he must make sure not to daven Mincha after plag. If he follows the Chachamim and davens Mincha until shekiya, he must make sure not to start Maariv until after tzeis.
13. One may not daven Mincha after plag, like the Chachamim, and Maariv before tzeis, like Rebbi Yehuda; that is an inherent contradiction [תרתי דסתרי] (שו''ע שם ).
14. Similarly, if one davens Maariv after plag, like Rebbi Yehuda, he must make sure to daven Mincha before plag (שו''ע שם ).
15. Similarly, someone who davens Mincha after shekiya must make sure to wait until after tzeis to daven Maariv to avoid contradictory practices.
16. Two days. Some hold this is even true on two different days, i.e., one should not follow the Chachamim on one day and Rebbi Yehuda on the other, and certainly not on the same day (מ''ב סק''ו ). However, others are meikel. They hold it is only on one day that one may not be inconsistent, but on two different days, it is mutar. This is akin to the halacha of שני שבילין : when the shaila comes up twice, one time after the other, it is not considered contradictory (מחזיק ברכה להחיד''א סק''ב, תו רת חיים סק''ד, קה "י ברכות סי' א '). Many rely on this.
17. B’dieved. If one already davened in a contradictory way, even in one day, he is yotzei and does not need to daven again (שו''ע שם ).
18. Pressing situation. In places where people gather for Mincha and if they would not daven Maariv right afterward, no one would come back and there would be no minyan for Maariv, the poskim are meikel to allow both Mincha and Maariv after plag in this pressing situation (מ''ב סי' רל''ג סקי''א ). Still, it is better to make sure to at least daven Maariv after shekiya if they are able to (מ''ב סי' רס''ז סק''ג ). They should also repeat Krias Shema [and Sefiras ha’Omer (see Issue 132, par. 12)] after tzeis.
19. Shabbos night. Even those who are careful to daven Maariv after tzeis during the week may daven after plag on Shabbos night. Since it is a mitzva to add to the kedusha of Shabbos and one already took in Shabbos, he can rely on Rebbi Yehuda, that it is night with respect to tefilla (מג''א סי' רס''ז סק''א, מ''ב סק''ב ). L’chatchila he should daven Mincha before plag to avoid inconsistency, as mentioned (see Issue 95 at length).
20. Nevertheless, if one is late and did not manage to daven Mincha before plag, in a pressing situation he may still daven Mincha and Maariv after plag, as we are more meikel about this on Shabbos night (דרך החיים הובא במ''ב שם ). It is still preferable to at least daven Maariv after shekiya (מ''ב וביאה''ל שם ).
21. Women. Women who are busy taking care of the home are considered to be in a pressing situation and may daven in a way that is considered inconsistent, e.g., Mincha and Maariv after plag and before tzeis (כף החיים סי' רל''ג סק''ט ).
