two ftp servers behind AirPort Extreme router

You cannot port map the same port to two different IP addresses.

What you need to do is port translate.

So you have Server 1 with UDP port of 21 and TCP 21

Setup Server 2 with both UDP and TCP port of 2121 .. the port translation to 21 on the local lan side is fine.

If you made a typo and the TCP port is wrong in your post tell me.

Also you really should not expose FTP to the world.. it has zero security.. People use SFTP now.

You could also consider using one SFTP server and one FTP just for junk..

As a BTW.. I frankly am amazed you can get port mapping of FTP to work at all.. There is a general problem on Airport Routers with FTP.. which is another good reason for SFTP.

You can find info comparing ftp/sftp if you look around…

http://radinks.com/sftp/FAQ.php

I have to back up what LaPastenague said, ftp is terribly insecure & prone to receiving many attacks when exposed publicly, passwords can be vulnerable to dictionary based attacks. At the very least you need a good firewall to reduce the number of scripts that perform automated attacks & active monitoring of that.

SFTP can use ssh keys & be setup to ignore passwords. It's configured as part of the ssh server on OS X/ macOS.

The remote access toggle enables ssh & sftp, test locally with no ports open for it before you try external use.

P.S. I think Airport routers can also automatically announce your ssh connection externally - wide area bonjour. Meaning that port mapping is not required to get in from the outside world. LaPastenague will know more about that config than me 🙂.