I have a joint project with another author. The project is represented by the other author's agent, and I'm querying elsewhere for the rest of my work. In the query forms (which seem to be the norm these days) it asks whether I'm represented or have been represented by another agent. Do I check yes or no, and/or mention the situation in my initial query?
You check no.
You yourself do not have an agent (more on that to come.)
You DO mention that a project you worked on is being shopped by Agent X of Flex, Hex and Vex Literary Represtidigitation.
Now to the more.
Please tell me you have a collaboration agreement in place with The Other Author.
If you do not, please get on in place immediately.
The agent may say s/he represents the project, but when a publisher offers a contract, the project doesn't sign on the dotted line. You do. That means you need advice and guidance from someone who is Team YOU not Team Other Author.
Your interests are aligned but not identical.
This is even more true if you are NOT going to be part of the contract, if you're being treated as a contributor NOT a partner.
A lot of agents will brush off your concerns with "we've done this a lot and it always works out."
All I can say is: you have a contract to protect you for when things DON'T work out.
You don't want to find out anything the hard way.
Get a collaboration agreement, or a participation agreement in place NOW.