How do I keep logins on two confluence cloud instances seperated?

[Xenia Spott]

We encounter problems when we try to switch from one confluence cloud instance to another.

When a colleague logs out his user A from a Confluence cloud instance AI, and tries to log in as a user B on a Confluence cloud instance BI, he cannot login with user B. 
After emptying the browser-cache, login as user B on BI is possible.

In this scenario we use a MacBook and have already tried different browsers. Users A and B do not share an email-address. Both cloud instances are confluence only.

Is there a way to avoid emptying the cache every time when someone wants to switch to a user on another confluence instance?

Thanks for your help!

[Steve Thornhill]

Or ingconito window on the browser will work to log in with two accounts. 

[Jason Hudson]

Firefox now offers containers that allow you to do this within one browser.

[Dmitry Mironov]

Use different browsers for each instance.