When you toggling between screens for your QuickBooks account so you work using both screens will depend on what browser you're using.
For Firefox: Press Ctrl + N. (Note: You won't be signed in to the second window but will remain signed in to the first one. The second window will allow you to log in to the existing company so that you may work using both screens.)
For Chrome: Press Ctrl + N. (Note: If you go to QuickBooks Online in that window you will be signed into the existing company.)
If the issue persists, I'd recommend accessing your account using a private window and log in from there. This way, we'll know if the problem is browser-related. Sometimes cache and temporary internet files from previously visited websites can take up space and affect the system's performance.