I'm not really sure how it works under the hood, not being terribly familiar with how these login systems work, but you can basically (in your browser) click "a

I'm not really sure how it works under the hood, not being terribly familiar with how these login systems work, but you can basically (in your browser) click "add an account" and it will open a new tab and you can sign into the account. So right now I have 3 tabs open for gmail with 3 different accounts.