Old question, but it popped up in my feed so figured I'd answer for future viewers.
It's true that agents don't represent books, they represent authors, but publishers buy books, not authors.
For that reason, I can't see an agent shying away from a great novel just because two authors had written it. If they see it selling, they'll represent it because they'll want their cut.
Agents take a cut of the publishing deal. So, if both authors are unrepresented, the agent would want to represent both authors and take a cut of the publishing advance. E.g. if they sold the book for a £100,000 advance, they'd take their 15% cut and divide what remains of the advance (and future royalties) between the two authors.
Things could get complicated if each author had/wanted separate representation. Whichever agent secured the publishing deal would want 15% of the entire deal and the other agent might feel they were losing out. The agents would have to agree to the collaboration and come to an arrangement. That could make them pause for thought.
Once that book is sold, I imagine an agent would be happy to represent each author individually for their future projects.