I agree. I would not try to account for the fact that the vehicle is used by both businesses. If spouse A started using the van in 2019 and spouse B started using it in 2023, that's what you would put on the schedule Cs.
This assumes you are using the standard mileage rate and these are personally owned vehicles that are sometimes used for spouse A's business, sometimes used for spouse B's business, and sometimes used for personal use. If the vehicle(s) was/were owned by one of the businesses, or if you were using the exact expense method which includes separate depreciation, I think things would be a lot more complicated.
Of course, each business only deducts the miles that business put on the vehicle, you can't double dip on the mileage. And, whenever you sell the vehicles, you will have to account for the total business mileage from both businesses when you calculate any depreciation that must be recaptured.
