I think this is where discovery comes in but I’m not a lawyer so I’m shaky on the process. My understanding is that the two parties have to give over certain information to each other like who the witnesses are and evidence that will be used in the trial. So even if they did have an NDA there is still the possibility of the prosecutors being able to question them in a trial, if they were handed over during discovery as witnesses. If there’s a lawyer or someone more knowledgeable about trial law lurking around that can correct me if I’m wrong please do!