Complementarity of one-particle and two-particle interference

In a two-particle interferometer, one can study the variation of both single- and joint-detection probabilities as functions of the phase shifts of the beams. By combining the usual definition for one-particle fringe visibility vᵢ (i=1,2) with a reasonable proposed definition for two-particle fringe visibility v₁₂, we show that vᵢ²+v₁₂²{≤}1 or, equivalently, vᵢ{v}₁₂${≤}1/2. Some extensions are discussed.