Focus your answer on what you can learn about the candidate during the interview beyond what is listed on their resume. Let the interviewer know how you would make a choice between them. This is also a great opportunity to show how you go above and beyond filling the current open position.

"It can seem like a tough situation when you have two great candidates and only one opening, but it can actually be an opportunity. To decide who to offer the position to, I rely heavily on what I learn about each candidate during the interview. The one who seems to have the most confidence and passion for the job is the one I would choose. Yet I don't forget about or simply dismiss the other candidate, especially if they were also a really good option. I might try to see if we have any other openings they could be a good fit for. Or at the very least, I keep their information saved in my talent pipeline and keep them in mind whenever new opportunities open up. Someone I interview today might be the absolute perfect fit for a job that opens up six months down the road."