Code:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?yourdomain1.com [NC]
RewriteCond %{HTTP_REFERER} !^http(s)?://(www\.)?yourdomain2.com [NC]
RewriteRule \.(jpg|jpeg|png|gif)$ - [NC,F,L]
Use this on your both domains to allow hotlinking from each other.