No, a series diode will not help; just add a diode in parallel with the uncoupler coil, arranged so it does not conduct on the normal polarity.
If each half of the output can be controled separately, you may be able to use a single output for two uncouplers? Connect each in series with a diode, rather than in series with the common junction of the two diodes; eg. the diodes from the motor side of the diagram, but separately feeding two solenoids, as in the left side of the diagram.