If you've a front-end load balancer listenening on www.idp.company.com, and it can terminate the SSL session and forward to server1.domain, then you may not need to do anything.
If you need your Access Gateway to also listen on www.idp.company.com, then search the product documentation for "virtual host" for some details on how to set up virtual hosts in the Access Gateway. E.g. here and here. Note that these virtual hosts are in the "back end" of the access gateway. The "front end" is a single Apache instance with an SSL certificate.
If for some reason you need multiple SSL certificates, then see this thread.
Pearse