This is one of the most debated issue in Clash of Clans. Should you set you infernos on single-target or multi-target? The answer is pretty simple; you set one on single-target and one on multi-target. This way you can defend against any strategy and any troops. Up in master and champion league, chances are that you are attacked by several different strategies. You want to make sure that you can defend against all of them Below are the strategies that each inferno tower mode defends well against.
So, as you can see, they both have their pros. For example, if a player is looking to raid you with all hogs and you have two multi-target inferno, most likely he will skip your base. However, if a player has GoWiWi trained and sees you have two multi-targets, he probably will try to take you out. Therefore, it is best to have one of each, so that you can defend against all strategies.