In order to make the Apple smartphone work with two physical nano-SIM cards, the device has to be opened and an internal component has to be replaced, which should be manageable for crafty users of an iPhone 12. But due to Apple's restrictive software, the results of the modification are not as spectacular as they could be.
https://www.notebookcheck.net/YouTube-video-shows-how-to-add-dual-nano-SIM-support-to-the-iPhone-12.552735.0.html