Assuming you have the balls bouncing off the walls already, you must be checking for when the ball's direction should change, and then changing it. When it changes for the second ball, send the message. You'd probably want to make sure that it only happens with the second ball too. If you're not using classes, this should be easy. If you are using a class, add a boolean that remembers if the ball should send messages.

Don't post questions twice, I deleted your other post

Thanks for the quick response, any idea how to send OSC messages from Processing? I found a tutorial on it but i'm struggling to receive any OSC messages in pure data even though I've set it to receive from the same UDP port.

Moved here from Programming Questions, since OSC is a contributed library, so I suppose OSC HELP is one too...