The double spindle machining centres are widely employed in industry in order to increase the manufacturing capacity. Such a machine tool configuration is economically advantageous, for it allows to increase the productivity at the same shop floor area and at moderate investment. Practically the cutting productivity is often limited due to a dynamic instability in milling. The reason for the instability lies in interactions between a double spindle machine tool and a parallel milling process. This paper presents a simulation model that allows to predict stability charts for parallel milling and to investigate specific influences onto the process stability. For the simulation the real dynamic machine behaviour was used, which had been ascertained by means of an innovative multiple excitation technique. The simulation results revealed that the process stability is influenced greatly by the temporal relationship of the parallel milling processes. The simulation results were verified on a double spindle milling machine.