Hello, unique solution. 

Thank you for choosing QuickBooks as your accounting software. In QuickBooks, you're only allowed to open one company each time. Though, you can switch it from another account. Let me show you how:  

1. Click the Gear icon in the upper-right corner.
2. Select Switch Company. 
3. You'll see your email address or company name that represents your other account. 

To learn more about switching account, you can check out this article: How do I switch companies?  

If you have other questions or concerns, please don't hesitate to reach out to me. I'll be here to lend a hand. Have a great day!

@unique solution 

Open one account on the primary browser and the second one on the incognito/private browser. Should you have 3 browsers (e.g Chrome, FF, Edge), you may open up to 6 accounts at the same time.