[jroid]

Go here, accept the conditions https://www.google.com/accounts/MultipleSessions

On the right top menu, Switch Accounts, sign in to second account, (may need to accept conditions again). At this point, you will be able to switch back and forth, between the two accounts

[mattwdelong]

I've done that and have been using it - it has many limitations but it's a small step in the right direction.