May I ask what is the best way to present a query letter when the book has been co-authored? Does one author primarily take the literary reins and handle most of the communications? If bios are required, is it best to present a bio of the group or each individual?
You both sign the query, and you include a bio for each author.
If one of you is the designated point person (i.e. using your email rather than the other person's) that's ok with me, but I really prefer to email each author so that all communication is direct and clear.
Both authors are signed as clients to the agency.
And before you get started querying, make sure you have a collaboration agreement in place. A collaboration agreement covers the financial split, the credit on the book cover (stuff like whose name comes first), who is responsible for revisions, and what happens if one of the authors decides not to continue with the project before it's published.
I won't send a project on submission without those questions being answered and part of an agreement.
This does NOT apply to author/illustrator pairings for picture books in case you're wondering. Art and text are separate not joint contributions to the book, and each is covered by a different contract.
We see this mostly with academic books and non-fiction.
This is NOT how ghost writing or co-writing for hire books are handled either.