Protocol for two coauthors seeking agent?

Honesty is the best policy. You need to tell agents that it's a joint work. However, that doesn't mean that you both need to sign all of the letters, or that you need to include two SASEs (one for each of you); you'll be far, far better off deciding that one of you will handle the mechanics of contacting agents and publishers at the initial stage and giving the other one some other administrative task to balance the load (like printing out all of the copies of the query package).

RLB: There are several writing teams out there, some of which started out equally unpublished before teaming up to write and then getting published together. Our own Cathy C writes with a partner, and if I'm not mistaken they were unpublished and then sold a book together. Now they've published several books since. The important thing to keep in mind is that if the book is strong enough and marketable enough, the agent won't care HOW MANY people wrote it.