Steps to reproduce:
At this point, everything about notebook "B" works. Notebook "A" appears to work at first glance (e.g., you can run Python commands), but you cannot save the notebook or do anything which interacts with the original notebook app.
This is because we use a sing cookie username which is shared between the different instances of the notebook. Cookies cannot be set on a per-port basis, so that's not a fix.
Maybe we should change the name of the cookie to include the port number (or another random, instance-specific string)?
This is already open as #1459, and yes, we should have some uniqueness for the cookie that is distinct on a per-port basis.