Is it permissible for an employee to receive two salaries from two different workplaces, knowing that he is seconded from his first job to the second, and the nature of his work does not require attendance?

The employee is obligated to perform his work according to the contract and its terms. It is not permissible for him to combine two jobs if that violates either contract. Absence from work is not allowed except with the permission of the responsible authority. Furthermore, it is not permissible to receive a salary for work where the contract or its terms have been breached. Colluding with the direct supervisor to be absent without the approval of the employer does not justify continued employment, because it is a betrayal.