Torah Reading: Error
Question
What to do if Baal Kore ends aliya a Pasuk early and the person receiving the Aliyah begins the second Bracha and the kehila (while the person is reading the second Bracha) corrects the Baal kore
Answer
Shalom!
Thank you for your question.
There is actually no true halachic source for where, how, and when to divide the aliyot of the Torah reading. In fact, there are over 33 different recorded customs on when to do so. It may even very well be that the aliya divisions were set by the printers without rabbinic consultation at all.
The only halachic requirements when dividing up the aliyot is that each aliya must contain at least three verses and that one may not end an aliya within three verses of a “setuma” or “pesucha.” therefore if this was the case, one would have to stop reciting the bracha, and the ba'al koreh should continue the reading
As such, one may essentially divide the aliyot anywhere one wishes and one should certainly not make any corrections once the oleh has began reciting his concluding blessing other than the cases mentioned above.
That being said, the aliya divisions as printed in the Chumashim have essentially become universal and one should not deviate from it without good reason and in consultation with the synagogue rabbi.
OC 132, 137-139 and commentaries.