The nonlinear evolution of double-diffusive instabilities into a laterally heated stably stratified motionless liquid is studied through direct numerical simulation in a two-dimensional set-up. In this paper, we consider liquids which are initially stratified through a constant salt gradient. The stages of evolution of the intrusions and their spatial scales correspond well with those observed in laboratory experiments, A central process in the evolution is that of layer merging. A particular case of layer merging is analysed in detail and a new physical description of this process is proposed, to which we refer as ＇layer sandwiching＇.