Building on the classification of modules for algebraic groups with finitely many orbits on subspaces, we determine all irreducible modules for simple algebraic groups that are self-dual and have finitely many orbits on totally singular k-spaces (k=1 or k=2). This question is naturally connected with the problem of finding for which pairs of subgroups H,J of an algebraic group G there are finitely many (H,J)-double cosets. We provide a solution to the question when J is a maximal parabolic subgroup P_k of a classical group.