The managers at my first job job-shared. It was a fucking disaster. The Director had returned from mat leave and the only way she could get to go part time was to agree to it permanently.
So when my boss (who worked to the Director) wanted to come back part time, the Director said she would only agree a job-share and on a perm basis. They found some money to appoint a job-share partner for 2.5 days a week, with original post holder doing 3 days and they had half a day together in the middle.
They struggled to find someone with the right skill set who wanted 2.5 days a week so they appointed based on the 2.5 day element rather than someone qualified for the role.
So, both job-share partners had young children who started nursery to facilitate their return to work. In 18 months neither of them managed a full week, either due to holidays, child sickness or their own sickness. Clients got bored of not knowing who was meant to be working which day, the lack of consistency, there were rarely handovers between them because of missing the shared days more often than not, and so me and a colleague ended up doing their work as well.
Luckily for me I was able to jump several ranks due to the experience this gave me and I left soon afterwards. Clients started leaving and the organisation had to make redundancies. The second job-share partner was made redundant, at which point the boss discovered that she had started her own business and had been sneakily poaching clients. Original boss is still there nearly 30 years later doing the same job part time and is just waiting to be able to retire.
Job shares can work really well, but they aren’t easy to get right.