Is car Benefit in Kind 50% each when joint users?

A couple who are both directors and work equally in a limited company business contract hire a car through the company.

Can the BIK be split 50:50 between them?

They both take remuneration up to the top of the NI Employers' Earnings Threshold, so both have some surplus personal allowances that could be offset against the BIK.

Both drive the car.

If it can be split 50:50, would it make any difference in a slightly different situation where both were directors but one did the majority of the work in the company (i.e. one full time and one part time)?

Thanks