problem with vsftpd on a non-standard port

hello all,
i am trying to set up 2 different vsftpd servers behind the same
internet IP using port forwarding in my router. i have configured
server A's (192.168.1.2) vsftpd to listen to port 20002 and server
B's (192.168.1.3) vsftpd to listen to port 20003. then i set up
port forwarding in my (netopia) router to forward internet requests
on 20002 to 192.168.1.2 and 20003 to 192.168.1.3 . so far so good.
after doing this, i can indeed point my FTP client to the IP address
+ port number of the server I want and login without any problems.
BUT! when i try and transmit or receive any data (even a directory
listing) it does not work. i know there is a secondary, data port
used for this, but i never thought it was necessary to set up forwarding
for this as i've never had to before with any other ftp server and
also, it is generated differently every time. my goal is to be able
to ftp in to either server from any client anywhere so having to make
changes in my client-side router at every place i might want to
ftp from is not an option. any ideas? i've been working on this for
2 weeks and i'm stumped. any help or suggestions would be greatly
appreciated.

thanks,
Jason Boardman