Google has multiple login features once and disabled it due to some infrastructure changes. After quite a long time Google has made the multiple sign in feature work it seems. I noticed this feature is back once again to let you do multiple sign in with your Google account and services. I have tested this with Gmail and Google Apps account and found its working perfectly without prompting me to Switch Accounts.
Google Multiple Login with Google Apps
You may test it with your multiple gmail account now and if its not working then try to enable google multiple sign in feature. This is really a big relief for Google Apps users where they can login to their gmail account and google apps account in another tab at a same time.