The past few iPhone models have included dual SIM support, letting you run a physical SIM and an eSIM to simultaneously get two lines of service on your single device. With the addition of 5G to the iPhone 12 series, though, dual SIM support won't be available at launch.
An internal Apple training document leaked on Reddit says that 5G will not be supported when using dual SIM mode on the iPhone 12 and iPhone 12 Pro at launch:
"Does 5G work with Dual SIM?"
When using two lines in Dual SIM mode, 5G data isn't supported on either line and will fall back to 4G LTE. If customers are using eSIM only and are on a 5G supported carrier and service plan, they'll have 5G access.
Support for 5G is one of the major new features of the iPhone 12 series, so the fact that 5G won't be available for dual SIM users is disappointing.
There is some good news on this matter, though, and that's that MacRumors has viewed an internal Verizon document that says that Apple plans to enable 5G support in dual SIM mode with a software update later this year. Until that update arrives, Verizon customers using eSIM will have to remove their physical SIM if they want to use 5G.