I can see how a tab functionality can be a helpful feature!
Multiple windows is possible if you're using the QuickBooks Online Mac and Windows App. The tab feature on the other hand, is not possible in QuickBooks App. This is only available if you want to access QuickBooks on a browser. The windows tile feature allows you to toggle between windows and it refreshes the open windows while you work.
I definitely think it'd be useful, and that many customers would enjoy having this feature too. I'd love to have you send feedback to our product developers about this idea so they can look into it. Feedback like this helps them implement updates based on customer suggestions. You can send your feedback to our developers using this link: http://feedback.qbo.intuit.com.
Please let me know if you have further questions about QuickBooks App. Take care.