There is a multi-window mode in Nitro 11, but it requires you to change a registry key (or create it if the key doesn't exist). Make sure you inform support if you having an issue that you have it on, because it's not a default, and it's possible that your issue might be tied to that.
It's labelled as a preview in the 11.0.3.134 release notes: https://www.gonitro.com/support/documentation/release-notes
You're currently limited to two windows.
Since you're about to edit the Windows registry, please be super-careful, because there are no safeguards to keep you from doing something that is difficult or impossible to recover from without a reinstall. It's generally a good idea to make a backup of the section of the registry you are editing.
In this case, you only need to edit a single key:
Open the start menu, and type regedit.
You need to run it with admin permissions (right-click, run as administrator).
HKEY_LOCAL_MACHINE\SOFTWARE\Nitro\Pro\11.0\settings\allowMultipleInstances
You want to change that from a 0 to a 1.
In case the key does not exist, you want to create a new DWORD (32 bit value) called allowMultipleInstances (make sure the case matches).
Double-click on the key, and change the 0 to a 1.
Good luck!