Accession to the United States has occurred in several ways. Texas was added by legislation, and the annexation law was agreed to by the people of Texas.
New Mexico, Arizona territories were purchased by the US in accordance with treaties with Mexico. Alaska likewise was purchased from Russia. States south of the 48th parallel became part of the Union by treaty with GB.
Disputes between states are required to be resolved by the Supreme Court. That is a legal matter, not a political matter. Every state has a boundary, and both would have to agree to it in the event of secession. That is why any pretense at secession quickly would go to the Supreme Court.
“Accession to the United States has occurred in several ways.”
The subject was NOT accession to the United States as a U.S. territory. The subject was accession of a foreign republic, foreing territory, or an existing U.S. Territory as a State in the perpetual Union.