Apple id

Log in to the same Apple ID on both iPhones. click here ➡️ Sign in with your Apple ID - Apple Support

Do note, this will share all calls, messages, contacts and other data from both iPhones to each other. So unless you are the user of both iPhones, this is not recommended.

What are you trying to accomplish exactly?

Are you the sole user of both devices?

You can sign in on multiple devices with the same Apple Account.

Click on this link to learn how to sign in with your Apple ID --> Sign in with your Apple ID - Apple Support

1. Open the Settings app.

2. Tap Sign in to your [device].

3. Enter your Apple ID and password.

4. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.