Photos defaults to “JPEG on top” and has a feature in Edit to switch to the RAW on top. The file name Photos shows is always the JPEG, even when it’s showing the RAW.
Nitro defaults to the RAW, though it shows the JPEG image, since that is what Photos provides to it. In contrast, it always shows the RAW’s file name. It also has a feature to switch to the JPEG when editing.
So, both can edit either half of the pair. Photos is JPEG centric and Nitro is RAW centric.
If you want to switch in Nitro, the button is to the right of the “four way control” and right below the histogram. It only shows up if you have an R+J pair in Edit.