The company will advise on this.
Similarly to any role on any team, that's up to each company and the hiring manager, and will vary based on industry, experience, role, geography etc.
Normally, your salary will be based on the number of days you and your partner work together and your individual split. Part of the consideration when working fewer days is the salary sacrifice that comes with it. Check out the The Salary Calculator for more info.
In a number of instances, insurance and other benefits would be based on your pro-rata. And actually, when working in a job share, the ability to do so in senior roles means, though there is a salary sacrifice, the earnings could still be very generous. But it's a personal decision - salary sacrifice, but you could keep career momentum, career stability, and gain more personal time.
Also, many Fortune 100 Best Companies to Work For offer health insurance for part-timers.
There are also up-and-coming companies who are offering company-like benefits targeted to the gig economy, which could potentially help offset any lost benefits.
More to come!