For the well defined process of 2s photoionization in atomicmagnesium with subsequent L1-L2,3M1 1P or 3PCoster-Kronig decay, exchange interference effects are studiedfor coincident two-electron emission. Both electrons areobserved for back-to-back emission in a plane perpendicular tothe photon-beam direction, changing the photon energy so thatthe photoelectron line is moved, from below to above, throughthe state-selected Coster-Kronig line. The coincidence signalof the singlet Coster-Kronig channel shows the expectedconstructive interference effect leading to an increase by afactor of 4. However, in the coincidence signal of thetriplet Coster-Kronig channel the expected destructiveinterference appears to be hidden due to (1P1-3P1)spin-orbit induced J-mixing. Implementing this complicationinto the theory which includes electron exchange and theinfluence of post-collision interaction, very good agreement isachieved between theoretical and experimental data. This givesquantitative confirmation of the selected process of sequentialtwo-electron emission described in a one-step formulation.