Even if your company does not have a job share policy (few do) or formalized flexible work policies (most don't), there are likely flexible work arrangements negotiated on a person-by-person basis all around you. Perhaps even job sharing.
Your key will be finding those examples and linking how well they work to your job share. The second strategy that will help you is finding ways that job sharing aligns with the values of your company (ie. collaboration, work-life balance, teamwork).
On another note, one of the most encouraging silver linings of the pandemic is overnight work from home. With over 40% of employees working remotely, most employers are finding managing flexible workers much easier than they ever imagined, even those who are stressed with the pandemic and a lack of reliable childcare. And a majority of employees, never want to return.
However working remotely is NOT all flexible work practices and one of the downsides of this way of working (alone), is that often employees work MORE hours (from the pressure of the perceived perk they are getting), unable to power-up and power-down. However, the structure of job sharing means continuous 24/7 for employers while employees are able to rest, recharge, and return with better solution-driven results!
Secondly, the pandemic has made employers much more open to innovative, new ways of working. And it's very possible they will see the value in job sharing if they can see HOW it relates to your company.