Doing one’s own work during official work hours

If the second job requires leaving the workplace of the first job, then it is clear that this is not permitted, because it is breaking the conditions of the first job. The first job stipulates that he should stay there until this hour until this hour, and the Muslims are bound by their conditions. If the other job does not require him to leave the first workplace and will not conflict with it or delay it, and he is only doing something when he finds free time during his work, it seems that this does not do any harm, because so long as he is not busy during this time, then the time is wasted and not put to good use.