I've just received a very kind rejection on a full from an agent (Agent A) I was excited about. She doesn't feel she can represent my book, but has referred me to another excellent agent (Agent B), at a different agency. The problem is, I already have a query with another agent (Agent C) at the agency that Agent A referred me to. This agency, like most, requests that you only query one of their agents at a time.
What is the etiquette here? Can I query Agent B right away? Do I mention I have my query in with Agent C? Do I wait? What if I never hear back from Agent C and I'm just in limbo forever?