  The interaction between a Bloch and a N\'eel wall in the parametrically
driven nonlinear Schr\"odinger equation is studied by following the
dissociation of their unstable bound state. Mathematically, the analysis
focusses on the splitting of a four-fold zero eigenvalue associated with a pair
of infinitely separated Bloch and N\'eel walls. It is shown that a Bloch and a
N\'eel wall interact as two classical particles, one with positive and the
other one with negative mass.
