As people have said, Taiping Rebellion is the most easy way of getting two Chinese states.
It's more complicated than 'Taipings secure south', however. You definitely need copious amounts of foreign intervention to ensure a status quo - remember that, even before conquering the entirety of southern China, the Taiping were already hurriedly embarking on expeditions to attack Beijing and Tianjin. In a Chinese state system that puts so much weight on legitimacy (Mandate of Heaven and all that), 'legitimating' symbols of authority (e.g. the capital) will always have a high priority.
It's therefore not likely that the Taiping will be satisfied with the status quo unless the Qing receive strong backing from other powers, maybe Russia. But then again, a Qing supported by Russia will easily crush the Taiping so in turn, you need support for the Taipings as well, maybe in the form of the other Western powers.
Consistent backing from Western powers will be required throughout the 19thC to prop up the Taipings, as their militantly anti-Confucian ideology is going to cause a lot of instability and brain-drain from their realm (if factional infighting doesn't do them in as in OTL).