Google uses cookies to keep track of whether or not you're signed in to your account. In the past, your @my-domain.com account had its own sign-in cookie, separate from Google Accounts, so you could sign in to both accounts at the same time from the same browser.
Since Google Accounts through work, schools, or other groups have moved to a new infrastructure that allows users access to many more Google services, they use the Google Account sign-in cookie. This cookie lets you sign in with only one account at a time. If you need to sign in to more than one account at a time, learn how to use multiple accounts at once.
