Currently if you want to sign in with multiple Google accounts, you would automatically signout with one of the account. But now Google has rolled out simultaneous signing in of multiple accounts giving you ease and yeah less frustration 
Currently multiple sign-in provides access to some of Google products like: Voice, Sites, Reader, Gmail, Calendar, Code, whereas Docs support is coming soon.
Note that enabling multiple sign-in to Google accounts will disable offline access to Gmail, Google Calendar, and to bookmarks. So lets start how can you do it:
STEP 1: Login to your Google account and in “Personal Settings”, Click “Edit” link followed by “Multiple sign-in”
STEP 2: Turn it “On” that says: Use multiple Google Accounts in the same web browser
STEP 3: Now tick all the checkboxes in yellow box to confirm you understand how to use multiple sign-in
STEP 4: After you tick all the check boxes, “Save” option will become active. Click “Save” so that new settings can take effect.
STEP 5: Now when you are on Google products that supports multiple sign in, you can see a small arrow for a drop down giving you option to Sign in to another account…
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