When a crime is “held back,” it’s usually strategic and legal, and there’s something about it that distinguishes it from other acts committed in the same larger incidence.
For example, a woman named Darlene Routier was accused of killing two of her sons. Because of their ages, the murder of one of the sons, who was younger than 7, carried a death penalty, without any other “aggravators” (like rape or torture), because killing a child under 7 alone is an aggravating circumstance. He other son was about 18 months older, and too old for the death penalty to be automatically applied.
So initially, she was tried only for the son who was under 7. The plan was that if that trial was lost, or ended with anything less than life without parole, then she would be tried for the murder of the other child.
Last time I checked, she was on death row, but still going through appeals. She still hasn’t been tried for the murder of her other son, and won’t be, if her execution is carried out, or if the appeal commutes her sentence to life without parole. If she gets life with parole, she may be tried for the other murder, and the prosecutor will ask for life without parole. Even if she gets a second sentence of life with parole, if she is serving two of them, the chance that she ever actually gets paroled is highly unlikely, especially as she can be made to serve the sentences consecutively, not concurrently, which means that if she were paroled on one sentence, she’d just begin the other one. She’d have to win parole twice, after serving whatever it took to be allowed to apply for parole (15, or 25, or 40 years, or whatever), TWICE.
But the murders of her sons, while one incident, were two acts. So this is legal. If she’d won at trial when tried for the first murder, it wouldn’t have been double jeopardy to try her for the second murder. It WOULD have been double jeopardy to try her for manslaughter under a different theory of the crime if she’d been acquitted of first degree murder the first time.