English Translation
Too many cooks spoil the broth.
スープができそこなう is grammatically broken and sounds unnatural. 出来損なう is an intransitive verb, But the broth itself doesn’t become spoiled on its own.
Also, we have a very common proverb equivalent to this English sentence:
船頭多くして船山に上る（せんどうおおくしてふねやまにのぼる）
船頭 means “captains of a ship”. Directions from too many captains are resulted in chaos.
The proverb is a bit long, so we sometimes shorten it as 船頭多くして (e.g. それってさ。いわゆる「船頭多くして」ってヤツだよね。= Ah, that’s a typical situation of “too many cooks spoil the broth”.)