hotlinking FAQ
First, the RewriteRules you have will do what you want. They aren't the best way, they miss a number of potential cases, but, in general should protect you pretty well. I would suggest that you use what is in the FAQ over those. to add another domain to the list, just copy the line that says yourdomain\.com and replace yourdomain\.com with anotherdomain\.com
However, the
deny from all
allow from mysite1.com mysite2.com
This will cause quite a bit of havoc.
First, every request that comes into your site will require a DNS lookup. Secondly only the people surfing from the servers mysite1.com and mysite2.com will be able to see the content. Note: from the server, not some surfer sitting at home using bellsouth, only your web servers will be able to surf your sites (and depending on how your server is configured, if they are on virtual IP addresses, they would probably not even be coming from that IP. Definitely not what you want.