I talked to Master, and though he's open to playing with S, he's hesitant for several good reasons. Firstly, S really isn't that interested in him. I hadn't really thought about that, to be honest - I was too caught up in how sexy it would be to be used as a teaching/practice implement by two people, ha! But the chemistry/interaction between two Doms in a scene is vitally important - you don't want them arguing while the sub is tied up and waiting for them to work things out!
Then Master raised another possibility - M and her girlfriend, Z. M is a very close friend of Master's and a fairly close friend of mine; we just met Z this year so we don't know her as well, but she's still a good friend. Though I find them both attractive - especially Z! - I'm a little more hesitant about them, mostly because I'm not familiar with their sexual personalities. I know via hearsay and what she's told me that M is very subbish, but that doesn't necessarily mean that Z is a Dom; in fact, Master and I don't even know if M and Z practice BDSM. We know M likes it, but what if Z doesn't? There are a lot of unknowns.
We'll have to talk a lot more about this, of course. I don't want to rule out asking S to join us, but I think it's kinda sketchy to "court" multiple people at once. Once Master and I decided who we'd like to ask, we'll go from there.