I have a manuscript out with two A-mazing agents right now. Since sending them my MS, I've nearly finished my second novel, edits and all. Here's my agent "protocol" question: Should I let the agent(s) know, and ask if they'd like to see this second novel, also? Or just hold tight and wait until I hear back on the first MS? My gut says two books would only sweeten the pot. What think?
I would wait until you hear back.
If one of the agents offers, then they'll certainly ask you about other books you might have.
If one of the agents passes on this project but thinks you have potential, they'll encourage you to send future projects, at which time, you can query with the second book.
If, however, an agent passes on a project, I don't think knowing you have another book in the pipeline will make them reconsider the project at hand.
If the second novel is a sequel to the first, then you can address that with any agent who expresses further interest.
If both agents pass on this project, I would continue querying this project until you've exhausted your list before starting to query on your second project. (ETA: Given Quickbread's response on that, I suppose it's subjective.)