I think you need a few things for the background
- Some sort of separate histories
- Some sort of barrier between the cultures
- Possibly some reason for the higher culture to detest or ignore the lower
Perhaps an environmental requirement of the higher culture that is absent for the lower would work; the lower culture can breathe a more polluted or sulphurous air, but living in the more hostile environment has limited their development.
There is room for all sorts of classic story elements in this setup:
- Discovery of a conspiracy between the leaders of the primitives and the high-techs to maintain the division
- A sudden medicinal need of the high-techs which necessitates the high-tech protagonist crossing into the primitive world seeking ancient medicine (also mineral need, see Avatar)
- Star-crossed lovers from either side of the boundary
- A primitive who secretly enters the high-tech world disguised as a high-tech
The interaction would depend on the history of the division; it could be a source of simmering discontent on one or both sides, or it could be an old almost forgotten thing where both sides have essentially accepted it (at least the leaders have).
There is a lot in it for the leaders of the worlds to maintain the separation; the high-techs don't want to be swamped by primitives, or particularly interested in helping them. By contrast, the leaders of the primitives have an incentive to maintain a story that the high-techs are bad in some way, to rein in the curious and maintain their own status in the primitive land.
This is why the conspiracy is tempting; the leaders of the primitives can make a nicer life for themselves by holding down the progress of the primitives in exchange for creature comforts and medicine from the high-techs. It could even be the primary mechanism of the separation between the cultures.
Ultimately all stories about other worlds and other cultures are really an exploration of our own nature and our own culture, so I'd expect this to look a lot like any place on Earth where one group is subdued for the benefit of another, and some members of the subdued are cast in the role of enabling the oppression. Slavery, Aborigine/Indian reservations, Workhouses in Dickensian England, the Irish Potato Famine, corruption of politicians by the Mafia, etc etc.
A great theme to add confusion to any story about a conspiracy is false conspiracy theories, which both cloud the issues and add a general skepticism for any one theory.