The previous comment is very correct. Two players who know their respective champions and roles very well will most likely dominate their opponents.
However, there are some champion combinations that compliment each other very well.
The first one that comes to my mind is

When these two are on point, they are devastating.
Probably the best combo I can think of between these two is for Morg to place her Tormented Soil at an enemy's feet and mostly behind them. This will proc the mark for Jhin's Deadly Flourish, rooting the target in place, so he can begin chunking them with his auto attacks. Just as his root ends, Morgana can use her Dark Binding to extend the cc. This whole time Tormented Soil should still be at their feet burning away at their health, and dealing more and more damage as their health gets lower. If needed, Jhin could have also dropped a Lotus Trap at their feet, and the cc combined with the added slow and the further chunk of damage will definitely put them in range for Jhin's 4th shot to execute.
Of course, this is great for eliminating single targets who are out of position. The full combo is very oppressive in the early laning phase and will get Jhin fed enough that he won't really need as much set up.
Morgana's Black Shield is also very good for Jhin since his positioning is very important. If she can keep him from getting caught out, this will really help their laning phase.