I am an Ne-Si. The way I think that works is:
Si is like a huge store of info (of a subjective nature), imprints of actual sensory experiences, that are somehow catalogued whichever which way I assign internal significance during the experience. For example if I walk downtown, I cannot store all the details, so I make mental maps instead, based on shape, color, types of store, traffic, whether I care or not about the library, etc.
Ne makes connections and has this huge Si-store to work with. Also, Si checks that the random connections made fit with the Si-store of info. In a conversation with a person, I'll be collecting tons of info into the store especially about things I care about or Ne-generated ideas that I am pursuing. So either to be able to generate Ne-ideas or to be able to check them.
To me it's kind of Si->Ne->Si or Ne->Si
I'm not clear how Ni-Se would go? I think I've had weird moments of Ni at times, that don't feel great (ultra inferior I guess) and kind of claustrophobic. And at those times, it felt like it was really an empty Ni without any kind of Se-input if that is even possible. It felt like being unreal and seeing the future without that much clarity. So I kind of wonder how Se works in the normal Se-Ni dynamic of an Ni-dom.