How to use two phone numbers on one iphone

With 2 physical sim cards, only In China mainland, Hong Kong, and Macao, see this Apple article: Using Dual SIM with two nano-SIM cards

https://support.apple.com/en-us/HT209086

Otherwise you should use a physical SIM card and an eSIM.

See this Apple article: Using Dual SIM with an eSIM

https://support.apple.com/en-gb/HT209044

Carrier helps you set up an eSIM on the phone to supplement the physical SIM. Read how to use: https://support.apple.com/en-us/HT209044