You can use GerbView to perform this task.
To do this you can do the following:
- Load the negative power/ground plane file.
- On the right side of the screen you will see a list of available settings for the loaded Gerber file. In the list you will find a setting named "invert". Enable this option to create a positive display of the layer.
Sample screenshot after adding layer and inverting this:
Then load the second layer and move it to the top of the layer list.
You should now see something like this:
Finally, save the two layers to a single Gerber file using Save As option:
Press OK and select a file name for the new Gerber file.
Final result shown is below:
GerbView® is a Software Companions product, and you will find more information about it here: