I have more than 1 domain in my account and are at the same server, so i'd like to update it both with the ddclient script, thanks in advance.

You can list the domain name one one each line like shown below:

# /etc/ddclient.conf
daemon=60 # Check every 60 seconds.
syslog=yes # Log update msgs to syslog.
mail=root # Mail all msgs to root.
mail-failure=root # Mail failed update msgs to root.
pid=/var/run/ddclient.pid # Record PID in file.
use=web, web=checkip.dynu.com/, web-skip='IP Address' # Get ip from server.
server=api.dynu.com # IP update server.
protocol=dyndns2
login=myusername # Your username.
password=YOURPASSWORD # Password or MD5 of password.
MYDOMAIN1.DYNU.COM # Hostnames one on each line.
MYDOMAIN2.DYNU.COM
MYDOMAIN1.COM
MYDOMAIN2.com