Let me understand this. If you connect the MTH ITAD 'NO' terminal to the Atlas signal DIN#1 the signal works like you want (Turns red, but after a while goes yellow then green.), correct? If so place one ITAD and signal at each end of the stretch you want the signals to be and wire them up like that. Now to tie them together so they trigger at the same time run one wire from the 'NO' terminal of the ITAD at one end to the 'NO' terminal of the ITAD at the other end. In this way you are basically giving each signal the same dual trigger.
Do not run any wires between the Atlas signals. You only run wires between the Atlas signals if you want one to control the other for sequential signaling, which is not what you want.