How can 2 GBR files for a single layer be merged into 1 file?

With KiCad’s Gerber viewer you can back-export gerbers to a KiCad PCB, and during the export you can choose on which layer to put the Gerber info.

When you have created a PCB from your Gerber files, you can change the layer of items.

This will of course not create a full PCB, because there are no real footprints in gerber files, it’s only pads and flash codes. There is also no real netlist yet, but it can be re-created because the netlist is implied in the placement of the copper tracks.

Some time ago I made a tutorial for: