To enable multiple sign-in:

1. Go to the multiple sign-in settings page.
2. Sign in to your account.
3. Select On to enable the multiple sign-in feature and select the checkboxes to confirm that you understand how to use multiple sign-in. Read more about what you need to understand before using multiple sign-in.
4. Click Save changes.

 
To enable multiple sign-in on additional accounts:
Once you’ve enabled multiple sign-in for one of your Google Accounts, follow the steps below to enable multiple sign-in for additional accounts you would like to sign in to:

1. Sign in to a product that supports multiple sign-in, using an account that has multiple sign-in enabled.
2. Click your name or email address at the top of the page.
3. Select Sign in to another account from the drop-down menu.
4. On the page that opens, enter the email address and password for another account you wish to access, and click Sign in.

ที่มา: Getting started: Enabling multiple sign-in