Two families, go on road trip, and two gay men change their driving (both have driver's licenses), and another person drives when there is a traffic accident. How to share the maintenance fee?

you can take insurance in case of maintenance costs.

since both families are relatives and friends, don't care who pays. It's different to take insurance.

In principle, whoever drives the train is the responsible person. If it exceeds the insurance coverage, the responsible person should pay the part that exceeds the insurance reimbursement.