Legal analysis: A tripartite agreement is generally invalid if only two parties sign it. If the parties conclude a contract in the form of a contract, the contract is established when both parties sign, seal or fingerprint the contract. If a tripartite agreement is signed by only two parties, the contract has not yet been established and has no legal effect.

Legal basis: "The People's Republic of China and the Civil Code"

Article 143 A civil legal act that meets the following conditions is valid: (1) The actor has the appropriate capacity for civil conduct; (2) the expression of intention is true; (3) it does not violate the mandatory provisions of laws and administrative regulations, and does not violate public order and good customs.

Article 490: If the parties conclude a contract in the form of a contract, the contract shall be established when both parties sign, seal or fingerprint. Before signing, sealing or fingerprinting, one party has performed its main obligations and the contract is established when the other party accepts it. Laws and administrative regulations stipulate or the parties agree that a contract should be concluded in written form. If the parties do not use written form but one party has performed its main obligations and the other party accepts it, the contract is established.