Is it true that agents are less likely to say yes to queries for collaborated books? If so, why is that?
Well, I don't think it is, but I will say that two authors means twice the hand holding, twice the email communication (mostly) and twice the work.
Each author is a client of the agency, thus each author has to be signed to a representations agreement, fill out their tax forms, get their 1099 at year end etc.
And books with two authors don't earn twice as much money.
That said, I've never turned something down because there were two authors.
On YOUR end however, you're going to want to make ironclad sure you've got a collaboration agreement in place before you do anything permanent like sign a contract.
Sorting out credit, payment, and promotion responsibility is something you want to discuss sooner rather than later.
I know that three-author anthologies are increasingly common in romance, but that's not collaboration. That's three separate authors writing three separate novellas. Three agents involved too. The one time I've watched that kind of cirucus unfold was pretty entertaining but mostly ok, since all the agents were good at their jobs.
It can go south VERY quickly if one of the authors is repped by a goofball. I've heard tell of that too.
If you intend to query as a duo, you BOTH sign the query:
1 Kale Row
2 Kale Row
Please direct email to: Colin Smith
And you query from the email address you want the agent to use for the reply.