  We propose entropic measures for the strength of single-particle and
two-particle interference in interferometric experiments where each particle of
a pair traverses a multi-path interferometer. Optimal single-particle
interference excludes any two-particle interference, and vice versa. We report
an inequality that states the compromises allowed by quantum mechanics in
intermediate situations, and identify a class of two-particle states for which
the upper bound is reached. Our approach is applicable to symmetric two-partite
systems of any finite dimension.
