Annoyingly, I discovered yesterday that I could no longer use two gmail accounts at the same time in the same browser. Basically, yesterday Google linked all their services together under one central account with no option to login into two separate gmail accounts at the same time in separate tabs in the same browser. I looked for a fix online and in gmail’s settings but could not find any solution.
Finally, this fixed the problem in 10 seconds.
Thanks to Daniel Sedlacek’s G+ feed.