A scenario is proposed for the simultaneous breakup process of an arrangement of parallel viscous threads. At very close distances, threads will break zip via an "in-phase" mode, that is, droplets will be formed at the same axial positions along the threads. In the case of threads that are further apart, breakup will occur via all "out-of-phase" mode, that is, on neighboring threads droplets will be formed at axial positions that are shifted by one-half of a wavelength. The smaller the viscosity ratio between the thread phase and the continuous phase, the closer the threads must be to break lip via this in-phase mode. This scenario agrees with recent experimental results.