Sometimes, issues like this arise due to problems with the data on your system. I'm here to help you fix it.
First, I recommend refreshing your browser. If the issue persists, try logging out and then logging back in to your computer. If you still can't open a second window in the accountant tools, consider using a private browsing window. This will help isolate the cache, which is often the cause of unusual behaviors.
Here are the shortcut keys:
Mozilla Firefox and Microsoft Edge: Press Ctrl + Shift + N
Google Chrome: Press Ctrl + Shift + N
Safari: Press Command + Shift + N
If the problem continues, you can try using a supported browser that is available to you.