Is it permissible to combine working as a consulting engineer supervising contractors with working as a contractor for the same projects?

It is not permissible for an employee to work for another party during official working hours because the employee's time during their working hours belongs to their employer. They are a private employee (ajeer khass). A private employee is not allowed to spend the contracted time in a way that does not benefit their employer, as their services during that period become due to the employer.