Not sure about Comair, but ASA will take multiple jumpseaters. And as you stated, if all are filled in the back, the actual jump seat may only be filled by a Delta, ASA, or Comair pilot. As I understand Delta's policy, they will only allow the number of jump seats available regardless of the number of empty seats. So in other words, if they only have two in the cockpit, only two jumpseaters. Of course, Delta pilots have priority, ASA and Comair next. Hope that helps.