Can I be signed in to two accounts at the same time if they use different email addresses?
You cannot be signed in using multiple email addresses at the same time in one browser. Your best bet is to have each account open in a different browser or use your browser’s private/incognito window feature to sign in to an additional account.
If you'd like to be able to easily switch between different accounts, you can use the same email address to sign in to all of your accounts. For more information see How can I use the same email address to sign in to all of my Harvest accounts?