How can I make ftp to listen on its default port 21 and another port 2121?

You'll have to start antoher service that binds to the new port. I don't know what ftp server you are using but the general flow is like so:

1) Create another service
for example if I am using vsftpd then:
cp /etc/rc.d/init.d/vsftpd /etc/rc.d/init.d/vsftpd2121

2) modify /etc/rc.d/init.d/vsftpd2121 to point to a new config file.

3) copy and edit the new config file

for exmaple for vsftpd:

cp /etc/vsftpd/vsftpd.conf /etc/vsftpd/vsftpd2121.conf
edit the new config file so that ftpd bind to your new port.

4) start the new service
/etc/rc.d/init.d/vsftpd2121 start

and if you want it to start on boot
chkconfig vsftpd2121 on

Also if you are using iptables, don't forget to open the port.

HTH
Ken

What lines to edit in the new .conf file?

depends on the ftp server. What are you using?

for vsftp it is

listen_port=21

change the port number to whatever you like.