Yesterday, while studying, I thought it would be great to be able to communicate by telepathy during exams. It made me think about that too! (I almost did a thread for that :biggrin3🙂
If Willow had started it, I would have understand it better.
We've already seen that Willow could speak in people's head (cf Bargaining). I guess with her new magics, and as everything is connected, she has a deep connection to the Scoobies (or maybe she just can read minds as well?). If my theorie is right, Buffy would just have to concentrate to 'call' Willow so that Willow would get it, her mental attention would be grasped.
So it kind of makes sense that Willow is able to have a telepathic conversation with Buffy and Xander, and hearing both of them.
I understand less how could Xander hear Buffy's thoughts and Buffy hear Xander's thoughts, unless Willow "connected" them as well.