Last thing, all you need for your system to start is the power-good signal through the ATX24 cable. And yes it requires a common ground for your PSUs to send this signal.
But, you do not need this so called ''Vantacor Dual PSU Adapter Cable 24 Pin 2-way'', save your $$$ because all electrical devices classified as CATII and + are supposed to be grounded on their case. So PSUs ground are attached to their case and motherboard ground is attached to the computer case
via its screw. So if both(or more) PSUs are in the same computer case ex ''Antec eighteen hundred'', you do not need a special cable, if they are not, you could take a cable, any cable grade, ex: AWG24, AWG14, etc, and solder/screw it to the second PSU case AND motherboard base
via one of its screw. as I've mentioned, you do not need a spacial grade of electrical wire since there wont be any current in this cable, but their respective ground will be the same.
you do not need to care about the second psu power-good signal, your motherboard will receive the first one, and will start.
you should wear safety boot and glasses when doing your experiment on PSUs since AMP can be very high on output side.
sorry if my English is bad
Ben,
Student Computerized System Tech