/etc/services - one service, multiple ports?

[herbert langhans <herbert.raimund%langhans.com.pl@localhost>]

Hi List,
I changed the ftp-port in /etc/services to 6021. The simple reason 
is to avoid password scanning on port 21, it works well.

One of the ftp users seems to get handicapped by some firewall or bad
ftp-client and cannot log in on port 6021. 

Is it 'legal' to have multiple ports on one service? This is the current
version of /etc/services:
<snip>
115 ftp-data         20/tcp    # File Transfer [Default Data]
116 ftp-data         20/udp    # File Transfer [Default Data]
117 ftp              6021/tcp  # File Transfer [Control]
118 ftp              6021/udp  # File Transfer [Control]
119 ssh              22/tcp  # SSH Remote Login Protocol
120 ssh              22/udp  # SSH Remote Login Protocol
</snip>

I would change it to:
115 ftp-data         20/tcp    # File Transfer [Default Data]
116 ftp-data         20/udp    # File Transfer [Default Data]
    ftp              21/tcp  # File Transfer [Control]
    ftp              21/tcp  # File Transfer [Control]
118 ftp              6021/udp  # File Transfer [Control]
118 ftp              6021/udp  # File Transfer [Control]
119 ssh              22/tcp  # SSH Remote Login Protocol
120 ssh              22/udp  # SSH Remote Login Protocol

Will the ftp-server attach to 21 as well to 6021 (not the same user on
both, maybe user A on 21, user B on 6021 simultaneously).

I didnt dare to change it yet, dont want to leave the server non
bootable or such troubles.

Did anybody try such trick?
Thanks!
herb langhans