Just add another exclusion condition for your other domain:
RewriteCond %{HTTP_HOST} !^myotherdomain\.com$ [NC]
RewriteCond %{HTTP_HOST} !^mydomain\.com$ [NC]
RewriteRule ^(.*)$ http://mydomain.com/$1 [L,R=301]
This way, any request for "myotherdomain.com" won't get redirect, and any request for "mydomain.com" won't get redirected.