I have hit a hurdle when trying to enable dual configuration. I can't add two .sof files to the .pof file when "internal configuration" is selected.
In Device settings -> Device & Pin Options -> Configuration, make sure the configuration mode is set to Dual Compressed Images. In the Convert Programming Files dialog, add the first .sof to the SOF data in the Input files section. Then you'll be able to click Add Sof Page to add the second image. See this online training for details: