Line geometry of pairs of second-order Hamiltonian operators and quasilinear systems

We show that a pair formed by a second-order homogeneous Hamiltonian structures in N components and the associated system of conservation laws is in bijective correspondence with an alternating three-form on a $N+2$ vector space. We use this result to characterise these pairs up to $N=4$. We also show that the three-form provides $N+2$ linear equations in the Pl\"ucker coordinates which define the associated line congruence.