301 redirect one site in multi-site installation
In the root .htaccess file is a set of rewrite rules. You can add a rule to that to redirect to the new site. 

RewriteCond %{HTTP_HOST} ^www\.olddomain\.com
RewriteRule ^(.*)$ http://www.newdomain.com/$1 [R=301,L]


untested, but should be about right.