This situation is not unusual—in fact, it is quite common. Architectural firms may team up to pool their skills to go after a project. Many times this is done because one of the firms has experience in a project type that the other firm does not have. Some firms, especially large firms, will probably have someone registered in the state where the project is located, but decide to work with a local firm.
The arrangement between these firms can be formalized through a contract, in a MOU for a less formal agreement, or just a handshake. Usually only one firm will have the actual contract with the owner of the project. Sometimes firms will create a joint-venture, which creates a legal entity that binds the firms together. This entity can then enter into contracts under the joint-venture name.
As to liability, the design professionals that seal the documents (usually there is one RDP sealing the documents for each discipline) have ultimate liability, but when a lawsuit is involved, the plaintiffs will go after anyone that can be tied to that project. The professional liability insurers and the firms’ respective attorneys will advise the design firms involved, especially if they enter into a joint-venture.