Pretty much what the title says. With a PoD after 1954, make South Korea just as oppressive as OTL North Korea whilst still being (nominally) capitalist. Making North Korea as bad as its OTL self is optional.
Keep South Korea as authoritarian as it was under Syngman Rhee. It could turn full-blown fascist, especially if the US is concerned about it becoming communist.
If North Korea remains better off than South Korea, then communism might be more popular amongst anti-establishment Koreans.