Is it possible to create a second box in the tag panel for Genre which will list the genres in the selected files?

I have to apologize, my memory fades: ...
in fact, the dialog "Mp3tag Options/Tag Panel" does not allow to define a dialog for the GENRE tag field for the second time.

You have to manually change something in this file: "%Appdata%\Mp3tag\data\usrfields.ini"

For example, put in a new section:
[#5]
value=GENRE
name=Genre (edit field)
multiline=

Doing so the Tag Panel will get the additional dialog field.

See also ...
multiline comment field in the tag panel

DD.20180130.1311.CET