How do I open a second window in a client in the new portal?

You can use the New Window button in the Accountant Toolbox to open a second window,  mike+1.

When working in a client file, open the Accountant Toolbox menu and click New Window. This opens the same page in a new tab. See screenshot for visual reference.

If you’re not an accountant user and want to open another tab differently, right-click the browser tab and choose Duplicate or Duplicate tab.

From that second tab, you can navigate to another page in QuickBooks Online. For more details about using multiple windows, see this link: You Can do WHAT in QuickBooks? Multiple Windows.
 

Please contact me if you need any additional assistance. I'm happy to help.