First of all, if you want a light-hearted look into this very phenomena, rent a copy of The Gods Must Be Crazy.
The only answer is that it might be done if the high-tech society is religiously determined not to interact with the stone age cultures. But, realistically, could it be done without keeping the stone agers on a reserve with oversight? (Would even that work for very long?) I suspect not for at least these reasons:
Stone age cultures migrate. Eventually a group would come in contact with the high-tech society. What will you do? Exterminate them in the name of keeping the rest of the stone age culture intact? With each passing generation there will be more stone age people and therefore more contacts simply because the stone agers walk around. With each contact comes cultrual contamination.
Massive land use
High-tech cultures, no matter how populously small, require space. They need space for manufacturing, agriculture, entertainment, housing, etc. Most people think of a city and believe the "city boundary" marks the extent of the city. Nope, the outlying farms, logging, mining, hunting, etc. mark their boundary — and that's not including wind farms, solar arrays, and whatever else may be required to sustain the city. Your population needs to be sustainable which means field laborers (and their families), transportation (families), education (families), medical services (families), right up to politicians (families). A fully self-sustaing technologically advanced city is massive. Even sprawling. And what's to keep the stone agers from discovering that wonderfully abundant field of corn, or beating the snot out of your solar array because the protective glass makes great spear heads, or seeing your wind farm from twenty miles away? More contamination.
I don't know how you would suppress this basic human(oid) emotion in your high-tech people, but inevitably, somebody is gonna want what the stone age people have. It doesn't matter if it's land, resources, slave labor, or child sacrifice. It would be difficult with anything but an incredibly small high-tech society to keep track of everyone who might think they have an interest in what the stone agers have.
"... was a system of institutionalised racial segregation and discrimination in South Africa between 1948 and 1991."
A high-tech society that didn't develop biased or biggoted feelings toward their stone age neighbors would be miraculous. Sci-Fi sometimes tries to promote the idea of universal righteousness as the "obvious" evolutionary result of greater intelligence (as if intelligence had anything to do with wisdom or self-discipline), but in the end, "human nature" tends to win out. Stories that depend on this idea often become boring because we, the people on Earth today, simply can't relate to it. We naturally distrust anyone who isn't "just like us". Thus, apartheid is born.
High tech cultures have leisure time. Idleness being the Devil's workshop, some of that leisure time would eventually be used by some well-meaning but ultimately world-destroying activist who sincerely believes that advocating on behalf of his or her distant cousins is a moral imperative. This ultimately validates Jean-Luc Picard's quote from the TNG episode Symbiosis:
History has proven again and again that whenever mankind interferes with a less developed civilization, no matter how well intentioned that interference may be, the results are invariably disastrous.
Which only goes to show that the time between holding up a protest sign and the zombie apocalypse is a lot shorter than you might think.