I'm okay at it. Step 1 is to actually know things about the interests of people. To do that, you have to be reasonably good at one-on-one conversation: asking people open-ended questions, showing genuine interest in them, etc. That's how you find out what they're interested in in the first place. Once you've done that, Step 2 is just to mention it to each of them when they are both present, especially when you're introducing them. "This is X, they are a professional Y and also like to do Z in their spare time" works well if you already know the other person is interested in Y or Z.