Can only one person be convicted of conspiracy?

The crime of conspiracy occurs when a set of people agree to break the law at some time in the future.

Assume that two and only two adults of sound mind and mental ability, acting entirely alone, have engaged in some activity which the state alleges is conspiratorial, and so calls for a trial. Both the prosecution and the defence agree that the defendants acted without involvement of any third party (but of course, the defence denies that the act itself was criminal, or disputes certain aspects of the prosecution’s version of events). Since at least two people are required to make a conspiratorial agreement, does a guilty or not guilty verdict for one of the defendants necessarily result in the same verdict for the other? That is, if the first defendant was found not guilty, would this verdict serve as an absolute defence for the second defendant? Or is there some set of circumstances in which one defendant could be found guilty but the other one not guilty?

(I’m sure answers will vary with jurisdiction, so please state yours when replying.)

If they are tried separately, the verdict will be up to the jury in each case. It is entirely possible that one person may be convicted and the other acquitted.

I suppose, but if the acquittal comes first, could the second defendant not use that fact as an absolute defence? As in, “Your honour, I am charged with conspiring with Fred. However, the court record clearly states that Fred was not guilty of conspiracy, and since two people are required to commit conspiracy, and the prosecution does not allege that I acted with anyone but Fred, I move to dismiss the charges.” Likewise, if the first defendant is convicted and the secon defendant is acquitted, could the first defendant not use the very same argument to successfully appeal his verdict?

One way for only one person to be charged and convicted for conspiracy would be if the other alleged conspirators were not available to be charged and convicted, e.g., they were all dead, or they were all in foreign jurisdiction(s) which would not allow them to be extradited for the conspiracy.

The common law rule is that conspiracy requires a “meeting of the minds” and at least two guilty parties, so if you had three conspirators and two were acquitted, the third could not be convicted (unless a fourth was on the loose somewhere). This is called the “bilateral” approach to conspiracy. The Model Penal Code is different and uses a unilateral approach, so that a single person who enters into an agreement with another to commit a crime can be guilty of conspiracy. Most jurisdictions have adopted the MPC unilateral rule. The federal government was still using the bilateral rule last time I checked, but that’s been a while.