  Frequency-locking and other phenomena emerging from nonlinear interactions
between mechanical oscillators are of scientific and technological importance.
However, existing schemes to observe such behaviour are not scalable over
distance. We demonstrate a scheme to couple two independent mechanical
oscillators, separated in frequency by 80kHz and situated far from each other
(3.2km), via light. Using light as the coupling medium enables this scheme to
have low loss and be extended over long distances. This scheme is reversible
and can be generalised for arbitrary network configurations.
