IRule Newbie needs help with simple redirection

You can't do it with SSL, sorry. The SSL key exchange happens before the BIG-IP can see the Host header and thus the BIG-IP can't know which certificate to present to the client. So unless you want to use one certificate for both sites (which would cause browser warnings) then you are out of luck. The only option is if you can get a wildcard cert (for *.mydomain.com for example). This is a limitation of SSL, not the BIG-IP.