Craft the first 2 paragraphs of your query letter as if your project had a single author. Pitch the hook for the collection and your interest in that particular editor/agent in the first paragraph, go into themes and topics of the collection in the second paragraph, then explain your co-authorship in the third paragraph: “This collection is a collaboration of Rosi X and Jane Doe. Rosi X is a published poet, with work appearing in [whatever]. Jane Doe is… We’ve each contributed half of the poems in the collection, and we plan to work together to support and promote this collection. Learn more about us at our websites [list sites].” Adapt that for your circumstances, of course. Both of you sign the letter.