Several questions regarding a murder trial and the accused and the accomplice.

Let’s say that two guys kill a woman. One holds her while the other shoots her. They both had a hand in killing her, except only one person can pull the trigger.
Can they both be tried for manslaughter?
If one is found to be merely the accomplice do they try the muderer first?
Does it matter who is tried first?
If the accomplice (the guy holding her) is found guilty can the trigger be found innocent? How bout the other way around, if the trigger is found guilty can the accomplice get off?
Can the testimony at the first trial get used in the latter trial?

Ex; Let’s say they both testify in the first trial (the trial for the accomplice) and the trigger says, “Yes, he held her and I pulled the trigger.” And the accomplice says, “Yes, I held her while he shot her.”
The accomplice gets the book thrown at him…can the trigger be found anything other than guilty at his subsequent trial?

In most, if not all jurisdictions, the guy who holds her and the guy who pulls the trigger are equally guilty. For example, there have been plenty of cases where someone gets shot during an armed robbery. The triggerman and the unarmed guy driving the getaway car face the same charge with the same penalty.

Separate trials could easily have completely different outcomes. It’s quite possible that the “trigger” could be found “not guilty” despite the previous conviction of the accomplice.

At ye olde common law, the guy who pulls the trigger is the “principal in the first degree” and the guy who was present and helping is an accomplice, the “principal in the second degree.” A person who was not present but helped (say, loaned him the gun knowing he was going to murder the victim) is an “accesory before the fact,” and a person who aided in escaping arrest or punishment afterwards would be an “accessory after the fact.” Conviction of the principal in the first degree is required for conviction of the accessories, so the principal in the first degree would have to be tried and convicted first. All accomplices are equally guilty of the crime committed by the principal in the first degree, except for accomplices after the fact, who a guilty of the less serious crime of being accomplices after the fact.

Most modern jurisdictions do away with all these distinctions in favor of the “law of parties,” which holds that all parties to the crime can be held equally liable (some jurisdictions still hold accessories after the fact are guilty of a lesser crime). Conviction of any one isn’t necessary for conviction of any of the others, and they can be tried in any order.

So, the driver of the getaway car is just as guilty of the guy who actually robs the liquor store, even if he never goes inside. I had a very, very, very hard time convincing a defendant that this was true just this week.

They could both be tried for murder. In the US, federal law (18 USC § 2) and the laws of pretty much every jurisdiction provide that someone who aids, abets, counsels, commands, induces or procures the commission of a crime is punishable as a principal actor. They could also both be tried for manslaughter, assuming the facts support manslaughter as a lesser-included offense.

The principle of *joinder[/i[ would usually suggest that they be tried together, unless there was some compelling reason to try them separately. Evidence that is admissible against only one of the accused can be admitted with a jury instruction that the evidence they are hearing may be considered only against the party it’s offered against. If the nature of the case or the evidence is such that evidence against one is so prejudicial against the other one that any probative value is outweighed, that might be a reason to sever the trials.

Even in a joint trial, one could be found guilty and the other not guilty; obviously this would also be possible in separate trials. The admission of testimony at one trial given during a previous trial would generally be a matter of hearsay – a statement offered to prove the truth of the matter asserted in the statement. It would not generally be admissbile at the new trial, but would be admissible under certain circumstances. For example, if testimony at the current trial contradicts the testimony given at the previous trial, the earlier testimony may be offered in evidence as a prior inconsistent statement.

Boy, this sounds oddly familiar.

Is there a legal precedent for a ‘domino’ effect in such court procedings?
IOW, if a case for the accomplice is built strongly on the testimony of the trigger, which leads to a guilty charge of manslaughter for the accomplice, can the trigger be found ‘not guilty’ at his trial? Can that trial’s outcome be used in the trigger’s trial?

It wil never be known who fired the gun in the DC sniper murder spree, but both parties were found guilty of murder: D.C. sniper attacks - Wikipedia

You got it. I’m having trouble with the DNA (blood samples left unguarded) problems in this case and the fact that the cousin may testify first, be found guilty, and then the actual killer could get off on a technicality.

Here’s quick glance at the story.

The problem that may come into play here is the admission of a confession against a non-confessing defendant. On the street, Abe says, “Me and Baker robbed the store.” Clearly Abe’s statement is admissible against him at trial. But is it admissible against Baker?

No.

In this case, the nephew’s confession is not admissible against the uncle.

Now, the nephew can certainly testify directly against the uncle.