Negative radiation pressure in case of two interacting fields

We study a simple toy model, which although probably does not have any direct physical applications, can serve as a nice pedagogical example for explanation strange phenomenon of negative radiation pressure and can also give some insight for understanding the interaction between radiation and a vortex. The model discussed is a classical field theory of two interacting scalar fields in 1+1 d. Without coupling one of the fields is governed by the ordinary $\phi^4$ equation and the second one obeys the Klein-Gordon equation. The possibility of existence of the {\it negative radiation pressure} with a respect to the mass ratio of those two fields is discussed.