Hang the two balls near eachother but with a small gap between
them. Blow air between them. Watch them come together.
The gap between the two balls acts as a contraction, locally speeding
the flow up and curving the streamlines. The local curvature
causes a pressure drop, resulting in a pressure difference across
each ball. This pressure difference will force the balls together.
Once the balls come too close, they will get blown out by the
stream as well.