Twin jet plumes on aircraft can couple, producing dynamic pressures significant enough to cause structural fatigue. For closely spaced jets with a moderate aspect ratio (e.g. 5), previous work has established that two coupling modes (antisymmetric and symmetric) are kinematically permissible. However, the dynamics of twin-jet coupling have remained unexplored. In this paper a more fundamental assessment of the steady and unsteady aspects of twin-jet coupling is attempted. While we document and discuss the nozzle spacings and Mach numbers over which phase-locked coupling occurs, our concentration is much more on answering the following questions: (a) What mechanism causes the jets to couple in one mode or the other? (b) Why do the jets switch from one mode to another? (c) Are the two modes mutually exclusive or do they overlap at the transition point? Our results reveal, among many things, the following. (i) For very closely spaced twin jets in the side-by-side configuration phased feedback based on source to nozzle exit distance of adjacent jets does not fully explain the coupling modes. However, the 'null' phase regions surrounding the jets where the phase of an acoustic wavefront (arriving from downstream) does not vary appears to correlate well with the existence of the symmetric mode. When the 'null' regions of adjacent jets do not overlap antisymmetric coupling occurs and when they do overlap the jets couple symmetrically. We provide a simple correlation using a parameter (a) that can be used as a simple test to determine the mode of coupling. (ii) The switch from the antisymmetric to the symmetric mode of coupling appears to occur because of an abrupt shift in the effective screech source from the third to the fourth shock, which in turn causes the 'null' phase region surrounding the jets to grow abruptly and overlap. (iii) The two modes are mutually exclusive. Our results provide considerable insight into the twin-jet coupling problem and offer hope for designing twin-jet configurations that minimize damage to aircraft components.