Thank you for wishing me luck, I do appreciate it :-)
I think the advice about discussing PDAs in advance is really helpful - I won't be arriving with either or them, or leaving with either of them and I've already made that clear - it seemed a lot more neutral to do things that way. I don't think I'd be comfortable with many PDAs myself, aside from taking their feelings into consideration, I'd also feel a bit exposed by putting our situation 'on display'. I'm not going to lie about being poly if someone asks me, but I'm generally a pretty reserved sort of person when it comes to my personal life, and being low key about it seems normal to me anyway.
As far as the conversation goes, I'm not going to bring up any relationship stuff, though I'd discuss it if they wanted to. I was hoping it would just be a chance for them to meet, realise that the other one is intelligent and reasonable, and be reassured to have put a face to the name. If they get talking on their shared interests (other than me) that would be even better! Because there will be some mingling involved, I will have time with each of them one on one, some time with both of them together, and they'll get some time alone if they want it. The whole thing will be pretty brief (2ish hours), which I hope will take the pressure off as well.
It's a little bit scary when you get what you want, I think a lot of my anxiety is coming from feeling like something has to go wrong, because right now things are so good. I really want to nip that in the bud before it becomes a self-fulfilling prophecy. The fear I keep coming back to is that X will change his mind when poly becomes 'real' to him - i.e meeting the other person, and/or that Y (for whom this is also his first poly relationship) will have a similar change of heart. Deep breaths!