Does the salary become unlawful if an employee continues to work for two competing companies without the knowledge of one of them, bearing in mind that the nature of the work is different and there is no disclosure of secrets?

If the first company is in competition with the second, then it is impermissible to combine the two jobs due to violating a condition of the second company. As for the salary, it is permissible unless your work in the second company causes harm to the first, in which case the first company has a right to compensation equal to the extent of the harm. This is as stated by Imam Ahmad and Ibn Qudamah regarding one who hires an employee, and that employee works for someone else: if the employer is harmed, he can claim compensation for the value of the harm; otherwise, not.