Thanks for posting your question here in the Community. I'm happy to provide clarification with having two or more books open on the same computer.
In QuickBooks Online, you can have several companies open on the same computer, but not on the same browser. Once you're logged into one of your QuickBooks Online accounts on a browser, this will default your browser to that account each time you navigate to and from the web page.
For now, you can open each company in a different browser or in a private/incognito window. You can refer to this System requirements for QuickBooks Online article as it has further information with the supported browsers you can use.
Feel free to comment below if you have any more questions. I'm always here to lend a hand and help. Have a safe and productive weekend!