Sounds like Se not being in your stack more than Si not being there in not being able to physically be present. You could be either intuitive or a sensor at this point. Though you're asking me for real life practical situations, could point towards a sensor.
It's more Si/Ne and Ni/Se, since they are the function pairs. They don't really act in isolation. I'm not very good at explaining them but here's a good example of how they work together.I'll try to give a bit more of a practical example of how that might work.Ni and Si are mapping/anchoring functions.
Ne and Se are exploration functions.
If we wanna use a metaphor: One holds the map, the other goes indiana jones style into new territory.
And they can't be described well without one another, so indeed they work in tandem.
- NiSe holds an abstracted map, but does literal exploration.
- SiNe holds a literal map, but does abstract exploration.
So you've taken on a new subject at college or whatever and you don't really know what you're doing. Mrs SiNe will look at what they've done before in the past and think "well I did Psychology in high school and this course is Anthropology" and Ne will generate all the ideas that they took from Psychology in High School, and then they actually did Biology as well so you could consider that, as well as a bit of History, and link it to Anthropology.
Mr NiSe will look at the Anthropology course and see it through their own frame of reference - Anthropology is this according to me and they'll go deep into the knowledge while Se will process the information they receive as is without really linking it back to anything in the past - Ni does that instead, but on a more abstract level than Si. Think papers being strewn about the place with your notes vs organising everything into a binder.
Si will go to the alphabetised section of the binder to find something - Ni will find it by kicking it with their foot and seeing that it's what they needed all along.