You may want to read this page before you go down that path (talks about redirecting one directory to another): http://www.webmasterworld.com/google/3205094.htm
I might use something like this in an .htaccess file on the domain that you want to send to the other domain:
RewriteEngine on
RewriteRule (.*) http://www.newdomain.com%{REQUEST_URI} [L,R=301]
You can leave off the "%{REQUEST_URI}" part if you just want to send them to the home page of the new domain.