  We study a planar motion of two hydrodynamically coupled non-identical
micro-swimmers, each modelled as a force dipole with intrinsic self-propulsion.
Using the method of images, we demonstrate that our results remain equally
applicable at a stress-free liquid-gas interface as in the bulk of a fluid. A
closed analytical form of circular periodic orbits for a pair of two pullers
and a pair of two pushers is presented and their linear stability is determined
with respect to two- and three-dimensional perturbations. A universal stability
diagram of the orbits with respect to two-dimensional perturbations is
constructed and it is shown that two non-identical pushers or two non-identical
pullers moving at a stress-free interface may form a stable rotational
equilibrium.
  For two non-identical pullers, we find stable quasi-periodic localized
states, associated with the motion on a two-dimensional torus in the phase
space. Stable tori are born from circular periodic orbits as the result of a
torus bifurcation. All stable equilibria in two dimensions are shown to be
monotonically unstable with respect to three-dimensional perturbations.
