It is impractical for two people working on a single project to have two agents representing that project. Virtually all reputable agents want the exclusive right to market the works of the authors they represent, and that exclusivity extends to joint works. If you and your collaborator have different agents, you'll have to come to some arrangement for representing your joint proposal. Many agents decline to work with other agents unless the other agent specializes in a different field, such as foreign rights or film rights.

If your agents are willing to work together, one agent will be designated the “agent of record” and will be responsible for marketing your project, but the two agents should split the normal commission (as opposed to each receiving the full commission). As always, anything you work out in these circumstances should be put in writing, and copies of the arrangement should be kept by you, your collaborator, and each of your agents. A written agreement is the only insurance you have in the event of a dispute over representation of the work.

Obviously, it's much easier to proceed with marketing if you and your collaborator begin with the same agent. Indeed, most collaborative works — especially those between strangers — are arranged through the agent. Typically, an agent will have a client or potential client who has a good idea for a book but who needs the services of a writer to create a salable proposal. She'll then match the client with a writer who is interested in the topic and available to do the work; the client and writer will discuss how to structure their working relationship and the agent will provide a collaboration agreement.

Before you leap into a collaboration with someone you don't know, try to set up a face-to-face meeting or at least a telephone conversation, to get a feel for each other and how you might work together. Collaboration is going to be much easier for both of you if there is some professional chemistry; if you think the two of you wouldn't work well together, it's best to wait for another opportunity.