I have a customer that wants to connect to FTPS via port 21 rather than the more standard 990. My thought was to add a 2nd listener configured for port 21, but when I have 2 listeners in there I get a message that states "configure default SSL". I don't see anything that says you can/should set a default of my two listeners. Any thoughts or comments on this are appreciated.
I could be a bit late on this one but...
It sounds to me like they want to use Explicit rather than Implicit, Services will allow this type of connection without changing the ports or adding another listener.
If you go to the FTP Listener in Services (Administration > Service Manager > Pencil Icon) you will see a section for Explicit SSL under the default listnerer, Just select your SSL certificate so it isn't using the default and restart the FTP Service.
To test your configuration you can use Filezilla (which is free) and put ftpes://YOUR_HOST
in the Host field then your Username and Password and set the port to 21. If everything is good you will see a popup with your certificate information click OK and try uploading a file.