一、共同犯罪的概念
依照我国刑法第25条第1款的规定，共同犯罪是指二人以上共同故意犯罪，此定义突出了共同故意对于构成共同犯罪的作用，是我国刑法中主客观相统一原则的具体体现。

二、共同犯罪的构成特征

构成共同犯罪，必须具备如下要件：

(一)主体要件

共同犯罪的主体必须是二人以上，具体来讲，可以分为下列三种情形：

1.两个以上的自然人构成的共同犯罪。这种自然人共同犯罪，要求各犯罪人都必须达到刑事责任年龄，具有刑事责任能力。

2.两个以上的单位构成的共同犯罪，即刑法理论中所谓的单位共同犯罪。

3.有责任能力的自然人与单位构成的共同犯罪，这在刑法理论中通常谓之自然人与单位共同犯罪。

(二)客观要件

共同犯罪的客观要件，是指各犯罪人必须具有共同的犯罪行为。所谓共同犯罪行为，是指各犯罪人为追求同一危害社会结果，完成同一犯罪而实施的相互关系、彼此配合的犯罪行为。在发生危害结果时，其行为均与结果之间存在因果关系。这种共同行为就其表现形式而言，可以分为三种情形：

1.共同作为、共同不作为、作为与不作为的结合。共同作为，即各共同犯罪人均实施了法律所禁止的行为而构成共同犯罪，比如甲、乙二人共同将丙杀死，共同不作为，即各共同犯罪人均未履行应当履行的义务而构成的共同犯罪，比如儿子、儿媳共同遗弃年迈无独立生活能力的父母。作为与不作为的结合，即共同犯罪人中有人系作为行为，有人系不作为行为，例如：铁道养护工甲与乙事先合谋破坏铁路设施，在乙实施破坏作为时，甲佯装熟睡，不履行其职责。

2.共同直接实施犯罪。在这种场合中，共同犯罪人没有分工，均直接实施犯罪的实行行为。

3.存在分工的共同犯罪行为。具体表现为有组织行为、教唆行为、实行行为和帮助行为。在这种场合中，各人的行为形成有机的整体。

(三)主观要件

共同犯罪的主观要件，是指各共同犯罪人必须有共同的犯罪故意。所谓共同的犯罪故意，是指各共同犯罪人通过意思联络，认识到他们的共同犯罪行为会发生危害社会的结果，并决意参加共同犯罪，希望或放任这种结果发生的心理状态。其特征是：

1.共同的认识因素，包括三个方面的要素：一是认识到不是自己一个人单独实施犯罪，而是与他人互相配合共同实施犯罪，二是不仅认识到自己的行为会产生某种危害结果，而且也认识到其他共同犯罪人的行为也会引起某种危害结果;三是各共同犯罪人都预见到共同犯罪行为与共同犯罪结果之间的因果关系。

2.共同的意志因素。其中，共同希望危害结果的发生，是共同直接故意;共同放任危害结果的发生，是共同间接故意，在个别情况下也可能表现为有的基于希望，有的则是放任。

基于上述理解，下列几种情况均不能成立共同犯罪：(1)同时犯不是共同犯罪。所谓同时犯，是指没有共同实行犯罪的意思联络，而在同一时间针对同一目标实行同一犯罪。(2)同时实施犯罪而故意内容不同，不构成共同犯罪。(3)超出共同故意以外的犯罪，不构成共同犯罪，此种情况，在刑法理论上称做“实行犯过限”的行为。

A joint trial of codefendants (also known as “joinder”) occurs when a judge merges the cases of two or more defendants. Joint trials happen when the issues in the defendants’ cases overlap enough to make a single trial both fair and more efficient.

When Joint Trials are Appropriate
Defendants don’t have to face the exact same crimes for a judge to join their trials. Rather, all that’s necessary is that the charges relate to substantially the same facts. Joint trials are commonly ordered in complex prosecutions for conspiracy, fraud, or terrorism, where multiple defendants are accused of committing a variety of crimes stemming from one set of facts. Some famous examples of joint trials include the Enron fraud case, the organized crime prosecution of John Gotti and his mob family, and the conspiracy and espionage trial of Julius and Ethel Rosenberg.

In contrast, judges may not join the trials of defendants who simply happen to be charged with the same crime. For example, a judge can’t join the trials of all armed robbery cases on the docket unless those cases involved the same set of facts.

There is no constitutional rule governing joint trials. Judges have broad discretion to decide whether to join trials, and will weigh a variety of factors when making this decision.

Factors supporting joint trials
Usually, judges will join trials only if the prosecution has charged the defendants together. However, there is no absolute rule forbidding joinder when defendants are charged separately, as long as certain conditions exist, including:

the charges against the defendants are based on the same evidence (such as when a group of defendants are charged with robbing a bank, but only one of the defendants is charged with assaulting a bank teller)
the defendants allegedly participated in the same acts or transactions (for example, when federal agents raid a meth lab, and one defendant is charged with manufacturing the meth and his cohort is charged with distributing it)
the crime allegedly involved a common scheme, conspiracy, or enterprise (as when a group of corporate officers are charged with a conspiracy to defraud investors), or
the charges are so closely connected in time and place that proof of one charge would require proof of the others (for instance, when one defendant is charged with robbing a bank and the other defendant is charged with being the getaway driver).
Factors against joint trials
The primary consideration in determining whether a judge has properly joined trials is fairness. Judges typically won’t (or shouldn't) join trials if none of the circumstances above are present. Judges also shouldn't join trials if the codefendants' strategies are antagonistic; in that situation, a joint trial would be unfair. Defensive strategies are considered antagonistic either when

each defendant asserts his own innocence and accuses the other of committing the crime, or
the judge or jury can’t believe one defendant without disbelieving the other(s).
The Joint-Trial Preference
Judges usually like to hold joint trials for several reasons. These include:

reduced workloads and
conservation of time and resources (avoiding repetition of evidence and the selection and maintenance of multiple juries, and reducing the financial and emotional cost of transporting and presenting witnesses).
However, these factors bow to the defendant’s constitutional right to a fair trial. If a joint proceeding impedes that right, then the judge must sever the trials.

Severing Joint Trials
Either the defendants or the prosecution may request that a judge sever a joint trial. Some states have laws requiring severance in particular limited circumstances, such as when multiple defendants are charged with capital murder. But as a general rule, neither side has an absolute right to a separate trial.

To convince a judge to split a trial that’s already begun, the defendant must establish compromise of the right to a fair trial. This right is compromised when evidence is admissible against only one defendant and is unfair to the other defendant(s). For instance, suppose Defendants Joe and Sam are charged with kidnapping a child for ransom, but only Joe is charged with sexually assaulting the child. A judge would likely rule that a joint trial of these defendants should be severed because the evidence of sexual assault wouldn't be relevant to whether Sam participated in the kidnapping; all it would do is enrage the jury so that they are inclined to find him guilty.

However, even when evidence that’s admissible as to one defendant is inadmissible as to the other, the judge need not sever the trials if jury instructions, witness admonishments, or other curative measures can fix the problem. Consider the trial of codefendants Ralph and Bob. If what Ralph knew about Bob’s tendency to fib is relevant to the case against Ralph, the judge might allow Laura to testify about times she spent with Ralph and Bob in which Bob lied. That evidence is probably inadmissible against Bob as an attack on his character. So, the judge could order the jury not to consider Laura’s testimony as it relates to Bob’s personality, and to contemplate it only insofar as it relates to what Ralph knew about Bob at the time of the crime. That way, the judge might preserve Bob’s right to a fair trial. (On the other hand, it might not, since that's a difficult task to ask of the jury.)

https://www.nolo.com/legal-encyclopedia/when-defendants-joint-trials.html