When two SIMs are inserted:


Adjust settings under Calling:

You can choose a preferred SIM to use for messages. When you do, you'll see it as the default in the text field of the Messages app 
To send a message from your other SIM, in the Messages app 

Choose how you want to handle mobile data if your preferred SIM can’t connect:


When two SIMs are inserted, you'll see them referenced in apps like Contacts, Messages, and Phone. The SIM name shows your carrier and, if you're using the Personal and Work profile, shows which it’s assigned to. You can rename the SIMs. You can also change SIM icon color to make them easy to differentiate.
