  We demonstrate quantum walks of correlated photons in a 2D network of
directly laser written waveguides coupled in a 'swiss cross' arrangement. The
correlated detection events show high-visibility quantum interference and
unique composite behaviour: strong correlation and independence of the quantum
walkers, between and within the planes of the cross. Violations of a
classically defined inequality, for photons injected in the same plane and in
orthogonal planes, reveal non-classical behaviour in a non-planar structure.
