Alternative to alternating relay? or help?
The datasheet for the relays says that these are intended for operation from an AC supply.  This is either 50 or 60 Hz.

You state that your pulse is only 20ms long and results in unreliable operation.  Increasing the pulse width increases the reliability of the relay but degrades the accuracy of your process.

You may now see where I'm going with this.  

Assume that the signal detect network for the control input to the relay uses a half-wave rectifier.  The control signal timing is asynchronous relative to the AC mains that the relay operates from.

Depending upon the phase of the AC mains supply when the control signal is initiated, the relay may not see an entire half-wave of the control signal.  It is entirely possible that the relay sees only two pulses: the end of one half-cycle and the beginning of the next half-cycle.

The datasheet does not mention a minimum control pulse width but I suspect that a minimum of two or three cycles of the AC mains is required for reliable operation.

In other words, the relay that you have chosen is most likely NOT suitable for your requirements.