  Bohr's Complementarity Principle is quantitatively formulated in terms of the
distinguishability of various paths a quanton can take, and the measure of the
interference it produces. This phenomenon results from the interference of
single-quanton amplitudes for various paths. The distinguishability of paths
puts a bound on the sharpness of the interference the quanton can produce.
However there exist other kinds of quantum phenomena where interference of
two-particle amplitudes results in a two-particle interference, if the
particles are indistinguishable. The Hong-Ou-Mandel (HOM) effect and the
Hanbury-Brown-Twiss (HBT) effect are two well known examples. However,
two-particle interference is not as easy to define as its single particle
counterpart, and the realization that it involves interference of two-particle
amplitudes, came much later. In this work, a duality relation, between the
particle distinguishability and the visibility of two-particle interference, is
derived. The distinguishability of the two particles, arising from some
internal degree of freedom, puts a bound on the sharpness of the two-particle
interference they can produce, in a HOM or HBT kind of experiment. It is argued
that the existence of this kind of complementarity can be used to characterize
two-particle interference, which in turn leads one to the conclusion that the
HOM and the HBT effects are equivalent in essence, and may be treated as a
single two-particle interference phenonmenon.
