I got this question recently:
“I have just completed my first script with another writer. We’re both unpublished writers, and I was wondering how you handle a query letter if there is more than one writer.”
What I do for scripts that I’ve written with other people is I write the query letter from my perspective. I sign it and I write as if I (not we) am talking to the producer. I’ll mention the other writer if it adds value to the pitch. If he worked as a policeman and the script is a police drama that should be mentioned. If he’s done stand-up comedy and the script is a comedy I might mention it by writing something like this, “My writing partner on this project, Joe Schmoe, has done stand-up comedy for years and is a member of the Groundlings in Los Angeles.”
In the query letter stage it’s really not that important if you have a co-writer or not so you don’t need to explicitly mention it unless mentioning it adds value to the pitch.
The producer might want you to sign a release form in which case all writers would need to sign it.
I usually include a brief synopsis with my query letter and I’ll list all writers on the synopsis at the top with the title of the synopsis like this:
You Screenplay Title
Joe Smith & Mary Jones
If you have any questions about how to write a query letter you should read my post How to write a professional query letter for your screenplay.
In addition, I’ve written many posts on query writing. You can find them all here: https://www.sellingyourscreenplay.com/tag/query-letters/.