Multiple instances of ftp-proxy

[Kevin]

I have a FreeBSD 9.1 box serving as a firewall using pf. The external
interface has multiple addresses, and I'd like each address to connect to a
different internal FTP server. I've configured ftp-proxy for the first
address, but can't find anything on how to properly start up multiple
ftp-proxy instances. Does anyone know how to do this, or point me to some
documentation on how to set this up? Thanks in advance.