Oh, you wanted us to tell you how.....
Here you go.
Go the briefcase in your QBOA (QuickBooks Online Accountant Version).
The very last menu item is New Window. If you click on that, it will open a new tab in your browser that is a duplicate of the one you are in now. You can move around to other screens within the same client database, no problem. You can also flip back and forth between the various screens, and open more than two windows if you want.
You cannot open the two tabs and be in two different databases.