I'm very interested in my wife having someone else too. Similarly, my focus isn't having someone else and she's not fully comfortable with the idea of me having someone else. She's not fully comfortable with the idea of another man having her either, but more so than the other way
Keep talking. My wife and I are talking a lot, and it's helping...even if we never do anything. It's been very erotic for both of us to just talk about it. Keep gently encouraging her, and let her know it's really ok for you. My wife is incredulous at times that I would want her to take another man, saying it's "counter-intuitive". She's right, it is counter-intuitive. But, that doesn't change the fact it'd be highly erotic for me to watch her having another man in her. She's logically got it that I'd really enjoy that, but emotionally she's not there yet. In time, maybe. Be patient, and keep gently encouraging. Even if nothing happens, you'll probably be closer from the experience of talking about it.