iPhone 12 Dual SIM 5G using a physical SIM card and eSIM will arrive later this year in a software update (via MacRumors).
iPhone 12 Dual SIM 5G
An internal Apple document was shared on Reddit and was confirmed as authentic. The relevant part says:
“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.
An internal document from Verizon though says it will arrive later in a software update. Verizon says eSIM customers have to remove their physical SIM card in their iPhone 12/12 Pro to receive 5G service.