Guidance / help required for stacking different focal length fits

This is possible yes, you can just assign the lights and calibration files from camera 1 to session 1, the other camera to session 2. Each camera will need its own calibration files so it's handy to have these in these sessions. Then you can let APP just go through all steps (go to tab 6 and press integrate). When you get 2 integrated results you can then clear the data in APP and load the 2 integrations as lights in the same session. You go to tab 6 again and press integrate and APP should be able to align the two and give an integrated result.