Why can another Apple phone receive SMS at the same time?

This is because two Apple phones use an icloud account at the same time, and they will be backed up and synchronized in the cloud when sending text messages.

iCloud[ai][kla？ D] is a cloud service provided by Apple, which allows users to store 5GB of data for free. On 2011May 3 1 day, Apple officially announced that it owns iCloud products for the first time. Apple claims that iCloud will replace MobileMe. ICloud completely rewrites the original MobileMe function, providing the original mail, calendar and contact synchronization functions.