There are several around here that are shared roads/lanes. Most of the time the guy in the back ends up doing most of the maintenance.
I have one farm that has a shared lane but it is spelled out where the easement is and who pays what in the deed. It has never been an issue as it is all in writing. It is a deed covenant and it is enforceable in court. If the one party does not want to pay the court can and will assess the property.
Was involved in that once with a new "City" land owner who claimed he did not have to pay as "HE" did not sign the original deed. He got to pay the maintenance cost and court cost. It got added to his property tax bill.
As for the split. Anything other than an equal split is just playing games and will just cause more problems. Divide the cost by 5 and go with it. The reason being is that the first fellow may not use the whole lane but he will also be the one that gets plowed out by the guy at the end.
Unless you are concreting or black topping the lane it,a 1000 foot lane does not cost that much to maintain in a year. Maybe $500-1000 in gravel each year. Some one will have to grade it every now and then. That can be a box blade on a tractor too.