  We theoretically and experimentally investigate the mutual collapse dynamics
of two spatially separated optical beams in a Kerr medium. Depending on the
initial power, beam separation, and the relative phase, we observe repulsion or
attraction, which in the latter case reveals a sharp transition to a single
collapsing beam. This transition to fusion of the beams is accompanied by an
increase in the collapse distance, indicating the effect of the nonlinear
coupling on the individual collapse dynamics. Our results shed light on the
basic nonlinear interaction between self-focused beams and provide a mechanism
to control the collapse dynamics of such beams.
