In this paper, we consider the mutual helicity of a pair of divergence-free
vector fields defined on, and tangential to, the boundaries of two disjoin
t domains in 3-space. Generalizing previous results of Moffatt and Laurence
& Avellaneda, we derive a simple formula for this helicity in terms of the
linking numbers of curves on the boundary of the two domains, and fluxes o
f the vector fields across corresponding spanning surfaces inside the domai
ns.