Yes, California is one, but is a minority with such regulations if not the only one.
"[B]ecause it is beyond question that sections 632 and 637.2 [of the California Penal Code] are primarily intended to protect the privacy of the communications of California residents, we conclude these statutes must be interpreted broadly to apply to all recordings of such communications—whether one-sided or two-sided. Any contrary interpretation would permit a business to maintain a policy and practice of recording one side of every telephone conversation with its clients without first obtaining the consent of all parties—a significant reduction in the scope of CIPA. (Gruber v. Yelp Inc., 55 Cal.App.5th 591, 269 Cal. Rptr. 3d 790 (Cal. Ct. App. 2020) (Gruber))
Even if interstate, California law would apply if at least one party to such a call is located in California. (Kight v. CashCall, Inc., (2001) 200 Cal. App. 4th 1377)
If a confidential call included only two parties and both agreed to record the side of the call of their own, and they voluntarily and in tandem or with mutual knowledge “sync” the two audio to create an audio recording substantially identical to the multi-way calls, I could not see a reason where a crime could be cognizable argued to be present.
Question 4 is less clear, I know of no case that decided whether it’s even possible to provide consent to recording of the other party or parties communications. In California, once an objectively reasonable expectation of privacy is not present for each party of confidential communications, it may be recorded without consent. Conversely, if one provides consent to the other party or parties to record their side of the communication, the communication will no longer be deemed private objectively reasonably based on Gruber. Therefore, the other side could record too. But this is “merely” a logical deduction and not an argument of authority — a court could decide differently.
Because of this lack of clarity, it is also hard to tell what decision would be made by a court in a case where a third party would do the syncing unless with the intelligent consent of the parties to an each-recording-their-own call audio recording. If each consensually provide a copy of their own side of the recording although unaware the third party could obtain a copy of the other side of the call could also be decided both ways, and even not obtaining their consent could not fully be clear, or could be deemed clear without the option for lenity as these scenarios rely on another apparently undecided question (whether it’s possible to consent to a one-sided recording without giving up confidential communication privileges).