Well, as owners of the vehicle they should be obligated to pay if no one admits to being the offender, and they can not prove that it was not themselves. Remove the vehicle or put both in jail for outstanding fines.

Or as an interesting experiment, decide that only 1 of them will take the responsibility of all fines and payment or punishment of them. I think as soon as responsibility of the fines is allocated and punishment is looming over one of them, they will pay.