Gmail users!: Check your “personal settings.” Google is currently rolling out the ability for you to be able to log into multiple accounts at the same time. If you tried doing this before, you would’ve had to sign out of 1 account before signing in with another. One way around this was to just use a different browser (ex. Firefox for one account, Chrome for another), but this was a pain. My account hasn’t updated yet, but hopefully yours has!
Gmail: Log Into Multiple Accounts At The Same Time