  The influence of the mutual interaction between the two outgoing nucleons
(NN-FSI) in electro- and photoinduced two-nucleon knockout from $^{16}O$ has
been investigated perturbatively. It turns out that the effect of NN-FSI
depends on the kinematics and on the type of reaction considered. The effect is
generally larger in pp- than in pn-knockout and in electron induced than in
photoinduced reactions.
  In superparallel kinematics NN-FSI leads in the $(e,e'pp)$ channel to a
strong increase of the cross section, that is mainly due to a strong
enhancement of the $\Delta$-current contribution. In pn-emission, however, this
effect is partially cancelled by a destructive interference with the seagull
current. For photoreactions NN-FSI is considerably reduced in superparallel
kinematics and can be practically negligible in specific kinematics.
