A good POD would be a KMT victory in the Huaihai Campaign in late 1948. Both sides are exhausted and before either side can muster strength, relations between the two superpowers take a plunge, forcing a ceasefire along the Huai River and Qinling Mountains (the same north south boundary when China is divided on that lines).
As Chiang recuperates, he realizes his plans to tackle corruption after the Commies are defeated are wrong-headed, and immediately centralizes military payment systems to undercut the warlords and boost public support. He then sends a token expedition force to Tibet (which he had always intended to) where the Dalai Lama retains de facto self rule while recognizing the RoC. Tibet itself is divided between the Dalai-controlled U-Tsang, the KMT-controlled Kham, and the Communist controlled Amdo. Stalin would still kill off his puppet leadership in Xinjiang and hand it over to the CPC, safe in Mao's ideological loyalty.
Meanwhile, the People's Republic of China is proclaimed with the capital of Beijing, declaring the southern half of China as "awaiting liberation". Both Chinas declare the other to be illegitimate puppets of the other superpower.
And finally, any Chinese Communist will eventually chafe at being a Soviet surrogate. There will still be a North Sino-Soviet Split. If (big if) Lin Biao or Deng Xiaoping take power after the split, the inner-Chinese detente could well happen by the 1960s. Chiang will also be careful to avoid being seen as a US puppet and will probably model his foreign policy after de Gaulle's. Unless Mao mismanages North China worse than in OTL (if that's even possible), a detente will still occur. The end of the Soviet Bloc will remove the last big obstacle to reunification, especially if both sides are poor one-party dictatorships of different color. By today the DMZ is replaced with a fence and both sides have in principle agreed to reunify. The confusing details are still an obstacle.