E.g. in an agency situation, where the causes of action arise from the defendant's agent's negligence/fraud, can the plaintiff directly sue both the defendant-principal & his agent? Since the agent would foreseeably be impleaded by the defendant-principal. Or must the defendant do that himself?
In a principal-agent situation:
- The agent is responsible for its own acts and omissions
- The principal is responsible for its own acts and omissions
- The principal is responsible for the agents acts and omissions that fall within the scope of the agency as reasonably known or assumed by the prospective plaintiff. It doesn't matter if the agent was exceeding their authority, the principal is still liable providing what the agent did was something the plaintiff could reasonably expect the agent to have the authority to do. For example, a company director is an agent of the company and ostensibly has the power to bind the company to anything - if they sell all the assets of the company to a third party that third party can rely on that ostensible authority even if the company required board sign-off so long as the third party didn't know that.
In general, a prospective plaintiff doesn't know who of the two might be liable and, more importantly, which has the most money/best insurance. If the matter reaches court the plaintiff would usually sue both which allows discovery against both and each to raise their defence - one of the defendants may fall away in this process.
The Plaintiff can only bring a valid lawsuit against (1) people who have legal liability to him, or (2) are believed to be among a group of people who in the alternative might have legal liability to him depending upon facts that could be disputed (i.e. it was either Abe or Betty or both, but you don't know which), or (3) if it concerns an interest in property and the defendant is someone who also has an interest in the same property (e.g. in a quiet title action you might have to name a co-owner even if the co-owner's interest wasn't disputed and the real issue was the validity of a lien on the property).
A Plaintiff cannot sue someone because they have liability to someone they did sue in the first instance (once the Plaintiff gets a judgment someone who was liability to a judgment debtor can be garnished by the Plaintiff).
Like all rules, it has exceptions. For example, some states allow direct actions against insurance companies that were casualty insurers of decedent's who allegedly committed torts while they were alive, by statute, even though usually the estate would be the defendant and the plaintiff couldn't sue the insurance company.