Why don’t two 50% opacity layers of the same color make the solid color in Photoshop

You can use Paint.Net with the PSD plugin (+ support thread) to open the PSD file, and then copy and paste individual layers to new documents.

^{Newer plugin releases may not work with old Paint.net versions. But you can try out older plugin versions.
For example the PSD Plugin v1.04 works well with Paint.NET v3.3.6}

The same objective can also be fulfilled with The Gimp, but it is a much bigger install than Paint.Net.

The simplest solution would be to use a mask, but that would require you to remember to make the mask just before you export.

If you want to go this route, select layer 3, and then ctrl for windows/or command for mac click the layer you want to be the basis for the transparency, and then click on the icon down the bottom of the layers panel with the circle in a square. This will give you the transparency you want, but it's entirely manual and you have to remember to update it when you edit the black layer.

You could create an action that selects the layer you want to use and makes a mask from that, but you'd still have to remember to run the action...