Go to addon domains in cPanel.
Add the domain you want. Where it says root directory or something like dat change the directory to the directory where the other domain stores it's content (most likely public_html)
Then for this domain you want to add change the nameservers to the same as the other domain.
Error from park wrapper: Using nameservers with the following IPs: 93.190.139.186,93.190.139.186 Sorry, the domain is already pointed to an IP address that does not appear to use DNS servers associated with this server. Please transfer the domain to this servers nameservers or have your administrator add one of its nameservers to /etc/ips.remotedns and make the proper A entries on that remote nameserver.