There is no such hard requirement. As long as the employer and the employee reach an agreement, the signature or seal of both parties is legal and valid.
According to the "Labor Contract Law"
Article 10 To establish a labor relationship, a written labor contract must be concluded.
If a labor relationship has been established but a written labor contract has not been concluded at the same time, a written labor contract shall be concluded within one month from the date of employment.
If the employer and the employee conclude a labor contract before employment, the labor relationship shall be established from the date of employment.
Article 16 The labor contract shall be agreed upon by the employer and the employee through negotiation, and shall become effective upon the signature or seal of the employer and the employee on the labor contract text.
The employer and the employee each hold one copy of the labor contract text.