Gmail and Google Apps have always been two separate entity for many people who uses Gmail as personal email account, while Google Apps serve as business or company email account. Previously Gmail and Google Apps are running two different system architectures and cookies, allowing users to simultaneously login to Gmail and another email account based on Google Apps on two different tabs within same browser window.
Good thing does not last long though. Google started to migrate all Google Apps hosted accounts to Gmail and Google Account platform. While the transition allows Google Apps users enjoy more services such as Google Voice, Blogger, Picasa Web Albums, and ability to sign in directly from Gmail homepage instead of having to go to custom Google Apps domain URL, it also means suddenly users lose some capability, including convenient mentioned above – it’s now impossible to login to Gmail and another Google Apps email account at the same time.
To end user of Google Apps after transition, both Gmail and Google Apps accounts now appeared linked. When user signs into one account, all existing Gmail, Google Account or Google Apps accounts on the same web browser will automatically switch to the newly logged in identity. Likewise, when logout, all Google Account, Gmail and Google Apps account in all tabs and windows of same web browser will automatically logged out.
In reality, the Google Apps and Gmail accounts are not linked, and remain separated. But as Gmail and Google Apps now using the same standard Google cookie to keep user signed in information after transition of Google Apps, the ability to login to both Gmail and Google Apps in parallel is lost, as the cookie will overwrite each other.
Google has probably anticipated the problem that will be faced by Google Apps users who have undergone transition, and has rolled out Multiple Sign-Ins feature which allows users to login and use multiple Google accounts at the same time within the same browser (on different tabs, of course).
The mutliple sign-in works for Gmail, Google Calendar, Google Sites, Google Docs, Google Code, Google Reader, Google Finance, Google Voice and Google Profiles. To enable multiple sign-in, just visit the multiple sign-in option and sign in with your main Google Account to turn on the feature.
Once multiple sign-in is enabled, click on your name or email address and select “Sign in to another account” to login to another Gmail or Google Apps account. The newly logged-in account will automatically have multiple sign-in support enabled. In fact, you can login to more Google Account as you wish.
My Digital Life has step-by-step guide on how to enable multiple sign-in of Gmail and Google Apps accounts.