Think about it. Phasing is matching up the ujoints at each end. With a double carden setup, there are two ujoints in the DC and they are phased to each other. The ujoint at the other end is left out there all by itself. There isn't anything for it to be in phase with.
That's why you need to have minimum deflection on that ujoint. No pulses generated by the DC, and (assuming no deflection) no pulses generated by the single ujoint.