A scenario is proposed for the simultaneous breakup process of an arrangement of parallel viscous threads. At very close distances, threads will break up 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 an 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 up via this in-phase mode. This scenario agrees with recent experimental results.