Band maths with SLSTR image

I am trying to do band maths with an SLSTR product. I can’t seem to get create a band using the expression S8_BT_in-S7_BT_in. Both products are open and they are on the same size image grid, but in different nc files. Should it be possible to do this?

Why do you open the images independently?
When you choose the xfdumanifest.xml the full SLSTR product will be opened any you can work more easily with the data.