Speaking with PayPal techs, they insist that a single PayPal Advanced gateway account should be able to accept payments on more than one domain name if using "Layout C" and the silent post method.
I have verified that I can indeed process a transaction... however, the silent post is (obviously) always sent to the url specified in manager.paypal.com... which, of course, is not going to work.
All I can figure is that since I run both domains, they think I should put a "forwarding" script in the hard coded silent post url which basically looks for an originating domain parameter and re-sends the post to the secondary domain.
If you have done this... I am interested to know how you did it.