Under California Rules of Court, a party in a civil actions has an obligation to file a Notice of Related Case per. Rule 3.300 Related Cases (b) Duty to provide notice.
Let's say you're the defendant in an unlimited civil lawsuit and you discover a related case that involves one of the other listed co-defendants who has a claim against the plaintiff, whereby the plaintiff in your case is the defendant in that other case and the co-defendant in your case is the plaintiff of the other case. The other case involves just those two parties and meets all the requirements of Rule 3.300(a) as referenced below:
A pending civil case is related to another pending civil case, or to a civil case that was dismissed with or without prejudice, or to a civil case that was disposed of by judgment, if the cases:
(1) Involve the same parties and are based on the same or similar claims;
(2) Arise from the same or substantially identical transactions, incidents, or events requiring the determination of the same or substantially identical questions of law or fact;
(3) Involve claims against, title to, possession of, or damages to the same property; or
(4) Are likely for other reasons to require substantial duplication of judicial resources if heard by different judges.
Now, although the other case involves claims arising from the same transactions/events, the case that you're in is like a joinder of parties where multiple causes of actions are included and new claims not mentioned in the other case are brought up in your claim as what may seem like reasons to justify a new separate claim rather than including it in the other case as a cross-claim (or possibly due to missing the opportunity to file a cross-claim). You discover that the plaintiff files in the other case that your case is a related case over there, but does not file the related case in your case to tell your case's judge about the other case.
Is it violating Rule 3.300(d), which is included below as reference, for failing to file the notice in both cases since it calls for notifying each case of each other?
(d) Service and filing of notice
The Notice of Related Case must be filed in all pending cases listed in the notice and must be served on all parties in those cases.
It seems reasonable to conclude based on the fact that since the same attorney is representing the plaintiff in your case and the plaintiff's role as a defendant in the other case, there's no doubt they filed the new lawsuit against you with full awareness of the other case still pending as they had appropriately filed the related case there within that other case, but failed to do so within the new cases against you. Rightly so, the other co-defendant would've also been aware of the other case and did not report the related case either.
What action, if any, can be taken by the defendant in this case, aside from filing themselves the Notice of Related cases as part of their duty after discovering of the case, that would serve to benefit you as the defendant? (assuming there was not an error regarding the determination of it as a related case)
Such as any sort of negligence claim against the plaintiff's attorney after summary judgement or a motion for dismissal of some sort? Perhaps a Request an order for Abatement, or Abatement Due to Another Action Pending, file a claim that the plaintiff's claim is improper for Splitting of Causes, Motion for Stay, file an objection by demurrer 430.10 (c) on the grounds that another action pending between the same parties on in part the same causes of action, or simply file the related case and let the court decide whether or not to join the cases on their end and then assuming the case is joined, what options are there then especially if there is suspicions for an abuse of process such as filing a frivolous lawsuit, could this be added as a claim to support that allegation?