I have a 58 with a manual hatch lift. I don't know for sure but I don't think I have the same wires you have.
Maybe it's not possible with a 58 or just much more difficult? I don't mind even if they were mechanical switches and non-electrical as long as they were on the inside so that when I'm car camping I can open either one...
My thought....... If you remove the rear hatch trim panel and find the unlocking switch wires (the one on the outside), there should be at least two wires leading from the external unlocking switch. There should be a plastic connector........... Use a screw driver or other object and manually lock the latch (with the hatch remaining open), disconnect the plastic connector and press the unlock switch, if nothing happens you have the correct connector.
The following is how I would proceed, but there is a chance of blowing a fuse, so proceed at your own discretion.
If there are only two wires, the switch is completing the circuit, to verify this......... manually lock the catch, on the body side of the connector (not the switch side) simply use a jumper wire to complete the circuit across the connector. It should unlock.
If there are 4 wires, use the center two.
If you get the lock to unlock using a jumper wire, you simply sister the two wires to a second switch.
Personally, I wanted both and I assume when I visited Eric at Ed Martins and I explained my set up, he did the same thing. My thought was ......... might need ventilation at night so pop the glass and needed a way to egress in case of emergency (my set up pops the glass regardless if the doors are locked or not). I also wanted to actually open the hatch from the inside not using a FOB.