I'm here to ensure you're able to switch to multi-user mode, Lempar.

May I know the exact error you've encountered when you add the second license? Adding more details will help us provide an accurate solution to resolve the issue. Also, attaching a screenshot would be a great help too.

In case you've experienced an error H202 or H505, here's what you'll need to do to resolve it.

To ensure that you've set up the multi-user network on your computer, please this article: Install Musti-user Access.

Additionally, I'm adding this reference to learn more about access limits in single or multi-user mode: Tasks that can be Performed in a Single or Multi-user Mode in QuickBooks Desktop. 

I'll keep an eye on your response, Lempar. I'm determined to get this multi-user issue resolved as soon as possible. Keep safe.