What if the Manchu Later Jin state and its Ming turncoat allies had been victorious against the armies of the Yongchang Emperor when they faced him at the Battle of Shanhai Pass in 1644? From that position they could have easily swept southwards and destroyed the Shun Dynasty in its cradle.
Great Shun, not yet in control of China south of the Huai River and not yet possessing much in the way of organized administration, which wouldn't come until the Tianyuan Emperor's reform, could not likely have recovered from a major loss, and certainly could not have raised another army in time to defend Beijing once the Manchu press their advantage.
If the Manchu destroyed the Shun Dynasty before it rose, how would they have dealt with the Southern Ming? Even divided and disorganized, it was some time before the Shun were able to effectively attack southwards, and even then, the Southern Campaign of Tianyuan Era was a gruelling, immensely bloody war lasting over 30 years and involving millions of people. The Manchu, like their Former Jin predecessors and all previous conquest dynasties save the Mongols did not at any point have the logistical capability to conquer the Southern Dynasties. In light of this, how long could a Northern and Southern Dynasties period with Ming and Jin have lasted? The period of Jin and Song may serve as an indicator, but that was the Middle Ages, this is early modernity and the Industrial Revolution is terminal by this point. Soon any regime in control of China is going to have to face the music. Shun faced it and learned from it, can a lasting northern chinese Manchu state? And what of Southern Ming?