shlishi,
So long as it is on different days (as opposed to the same day) it’s only an inyan rather than mandatory?
It’s okay b’shaa’s had’chak.
charliehall,
Or, on Friday afternoon, at the same shul! There are two shuls in my neighborhood where there is a maariv minyan before a minchah minyan.
(Somehow I don’t think this works ;).)
In theory, if the people who conducted themselves like R Yehuda, and the people who conducted themselves like the Rabbonn, would be davenig at separate minyanim, there shouldn’t be a problem.
In reality, I think most people conduct themselves according to the Rabbonon (anyone who davens mincha during the week after plag).