Suppose the one who is leining (reading) the Megillah needs to stop in the middle of leining, and is unable to continue. Is someone who was there the entire time allowed to take over?
Nitei Gavriel (38:9) says that it's fine, provided that (as in your example) the second person heard the berachos and the reading up to that point.
Although in 45:2 he cites differing opinions about the case where the baal korei was unable to continue reading and someone else takes over, whether that second person should start over from the beginning (which, he says - citing Shaarei Teshuvah, Orach Chaim 692:1 - is preferable if it's not too burdensome for the listeners, or if they agree to his doing so). I'm not clear on the reason for the difference: Magen Avraham there (the source for the stricter position) analogizes it to a case (284:5) where someone else had to take over the haftarah reading, where he has to start over so that the entire portion is bracketed by proper berachos, but then the same would seem to apply whether the takeover was planned or not.
The IDF handbook for Purim (available here) in question 30 footnote 62 on page 61 quotes Rav Chaim Kanievsky and Rav Mordechai Eliyahu as approving of the arranging for multiple Ba'alei Keriya even from the outset.
The Young Israel of St. Louis (Missouri) does this l'chat'chila every year, and has done so both under its current rabbi, Rabbi Moshe Shulman, and IIRC under its previous, Rabbi Jeffrey Bienenfeld, so I assume that at the very least yesh al mi lismoch (there's an authority to rely on for the practice). Moreover, I cannot think of a reason to forbid it (not that that means much).