Note: before you do this, you’ll need the phone with you that has already been activated for the “two-factor authentication”.
You will see the code in the Authenticator app on the first phone.
We had a customer that had already set up the Microsoft “two-factor” authentication on her phone.
She wanted to add her husband’s phone to the same Microsoft account without having to go through the many tasks required to generate a new QR code.
The way to easily do this, is to install the Microsoft Authenticator app on the second phone.

When you launch the app on the second phone, click the “Already have a backup” link at the bottom of the screen.
You will then be taken to the log in screen. Here you must log in with the same Microsoft account as the first phone.
Because two-factor is already set up and working on the first phone, you will be asked to authroise the second phone by way of typing a two digit number on the first phone and then following the prompts on both phones to add this second Authentication app to the Microsoft account. Sharing the same Microsoft account on both phones is the easiest method to share the codes from the first phone.

If you want to use another Microsoft account on the new phone, then you will need to turn off the “two-factor authentication” for every account that shows a code. You will then need to turn on the “two-factor authentication”.to the same accounts and then you can use both phones to scan the QR code given by the website.