Co-Author Query Letter Question

That depends. Is one of you clearly the "lead"? Does one of you have more experience or more connections? If so, I'd use that person as the contact. But I would definitely create an email address together so that both of you can be informed along the way. It will help with communication between the two of you and hopefully avoid misunderstandings. Good luck! ~Karen

Speaking as someone who has written with a co-author for years, I highly recommend you obtain a separate email for your joint writings. Make it at one of the sites that allow for large attachments (such as Google or Yahoo) and use a name that encompasses the partnership. Both of you should have equal access to it so you can check for responses separately and then forward rejections or acceptances to your separate emails.

You should also adopt a single P.O. Box for joint communication with publishers/agents. If you live in different cities, you just need to make sure the box is in the larger of the two cities for quick response times.