easiest way to add a second domain with email to a main multisite domain

We have a multisite domain with domain mapping. http://aresnyc.org

5 sites two sites have domains mapped to them via a parked domain. No dedicated IP.

We want to add a second domain to http://aresnyc.org . In other words if someone types in http://nycdistrictares.org it will load up http://aresnyc.org for SEO and non Dup purposes redirect to http://aresnyc.org and still have email with http://nycdistrictares.org Cpanel Account by the way

Also I did try multidomains plugin and it still does the same thing load the header and footer, No Content then stops loading heres the url http://aresnyc.org

add on domain is http://nycdistrictares.org – This is what the url looks like http://aresnyc.org/wp-signup.php?new=nycdistrictares.org