According to the great R. Meir Hakoheyn, a talmid of the great R. Meir of Rothenburg, in his Hagahos Maymonios, it is said that:
"If a city is all Kohanim, with no Yisroyel there, then it follows the holocho logic to deduce that a Kohen gets the first two aliyos to Toroh, and the rest should be read by women."
( see Mishne Toroh, Hilchos Tefiloh, 12:19)
Shulchan Oruch.-O.H. 141:2 Magen Avrohom forbids more than one person to recite blessings simultaneously, in order to ensure that every word of the brocho is heard clearly.
In one fo his rare rulings, R. Moshe Feinstein also ruled that two Bar Mitzvoh boys cannot read the maftir and haftoroh sections in unison (Igrois Moishe, O.H. I-102).
Sure, it feels good to speak Israeli Hebrew. To read Maqor Rishon, or Maqor Chayyim. To pronounceSav without a dagesh as Tav. To go from L'hoidois to L'hodot. Maybe it feels better to daven the local Sephardi pronunciation as well.
Eliezer Ben Yehuda, the authority of the modern Hebrew grammar, was so happy to be here that he, paradoxically, severely beat his family members when they slipped and spoke a word of Yiddish!
No support for family violence here, though. There can be no law that can make the holy Yiddish, the language of Golus, which still rules ever so painfully, the language of the great Be'er Hagolah, the Vilna Gaon, and Chofetz Chaim, a taboo.
The straight and honest halachah advises ashkenoizim to remain within their pronunciation. The saintly and revered, the Israel-loving R. Shlomo Zalman Auerbach who loved Israel, too, ruled so, in V'olehu Lo Ibul, Part 1, Opinion 44.