External electric fields were used to amplify interfacial fluctuations in the air/polymer/polymer system where one polymer dewets the other. Two different hydrodynamic regimes were found as a function of electric field strength. If heterogeneous nucleation leads to the formation of holes before the electrostatically driven instability sets in, the dewetting kinetics is not influenced by the electric field. Stronger electric fields lead to a spinodal electrohydrodynamic instability which causes the formation of polymer columns on top of the second polymer. The rapid column formation leads to a substantial deformation of the lower polymer layer at the column contact line, caused by the high viscous stress concentration there. With time, the deformation of the lower polymer layer is increased by the action of the interfacial electrostatic force. In addition, the analysis of the polymer-polymer interface during the early stage of the instability indicates a slip boundary condition for the upper layer on the liquid substrate.