This sounds a little unfair, as they appear to be valuing full-timers more than part-timers, especially as women are often usually the part-timers because of child care commitments. Some people might call this sex discrimination by the back door. I did a job share for several years and our job contracts were treated in exactly the same way as full time employees were treated. I would suggest your part-time colleagues approach their boss and ask to be treated on the same basis as full timers when overtime is being allocated. To be fair, the organisation may have felt that part-timers specifically wanted shorter hours because of their domestic commitments and might not have been interested in working overtime, so there may not have been any deliberate attempts are unfairness and if they approach the matter in that vein, they might be more successful in getting the policy changed.