In the past, I have davened at a Chabad on Simchat Torah. Their custom is to call up multiple individuals per aliyah rather than having multiple kriot. The only other places I have seen this was at the schuls of some of the less religious elements from my family, where they would call up typically a pair of spouses for an aliyah. What is the basis for the former practice, the latter being obviously outside the ken of Orthodox Judaism.
Nitai Gavriel (Hilchos Chag HaSukkos 98:10) brings this custom as existing in several communities. This was permitted because the joy in celebrating the completion of the Torah.
The Nitai Gavriel himself says that this should only be done in those communities where this is the established custom. Otherwise, one should be more strict, and if they need to do this due to time constraints, they have one person say the Bracha rather than everyone saying their own. (The larger issue here being several people saying a bracha simultaneously out loud, not so much having many called to the Torah at once).
(Perhaps the countervailing concern that justifies the custom is that everyone "called up" just standing around and saying Amein to the Bracha isn't really doing anything different than hearing Kriah all year round).