Can I use multiple URLs in the URL field of KeePass?

You can create a copy of a password entry:

And then only use references for username and password in the new entry:

Then, adjust the URL in the new entry.

The answer from Oliver Salzburg is very good.

Complementing it, if you already have an entry and want to link another entry, go to the entry, properties, then tools, insert field reference and link to the type you need.

See an example here:

Switch to KeePassXC. As of the 2.5.0 release, this now works as you would expect.