  We show that skyrmions on the surface of a magnetic topological insulator may
experience an attractive interaction that leads to the formation of a
skyrmion-skyrmion bound state. This is in contrast to the case of skyrmions in
a conventional chiral ferromagnet, for which the intrinsic interaction is
repulsive. The origin of skyrmion binding in our model is the molecular
hybridization of topologically protected electronic orbitals associated with
each skyrmion. Attraction between the skyrmions can therefore be controlled by
tuning a chemical potential that populates/depopulates the lowest-energy
molecular orbital. We find that the skyrmion-skyrmion bound state can be made
stable, unstable, or metastable depending on the chemical potential, magnetic
field, and easy-axis anisotropy of the underlying ferromagnet, resulting in a
rich phase diagram. Finally, we discuss the possibility to realize this effect
in a recently synthesized Cr doped
${\left(\mathrm{Bi}_{2-y}\mathrm{Sb}_{y}\right)}_{2}\mathrm{Te}_3$
heterostructure.
