Many people do have multiple Google Account where Google have never restricted anyone could have 1 account only.
It might be very annoying when you got to log into your account from different browser and type your login credential time by time. So, Google provided multiple sign-in feature to their users.
1. First of all, log into your Google account,
3. Select on the Edit button from the “Multiple sign-in”.
4. Do alert that the multiple sign-in will only work for Calendar, Code, Gmail, Reader, Sites and Voice only.
5. Turn on the setting as attached below.
7. Well done, you have done it.