There are many variables that are loaded with each individual team (color schemes, permissions, etc) so that it's necessary to logout and then back in to access your other teams.
However, you can use the "Private" browsing function in your browser to open two sessions at once. In Chrome it's called Incognito, in Firefox, Private Browsing, etc. You'll have two windows open at once, rather than tabs, but you can switch back and forth between your teams. Screenshot attached.
