Getting a ball to settle between two points.

I'm building a simple iPhone application and for it I need a ball to be attached to two ropes, each rope is in turn attached to one point. So essentially I want a ball to hang on a rope between two objects. The problem is these two objects will move around so the ball needs to settle between them. For this I have attempted to use joints and failed because the "Anchor" isn't hard and fast, it moves, so if I set the anchors of both joints to say "5, 0, 0" then it will pull it apart when the objects begin to move. Does anyone have any idea how I can solve my problem? I suppose rather than setting an anchor on a particular axis I want it to change constantly.