since you're using the standard mileage rate, the date in service doesn't affect the mileage deduction, but one must be entered because it's simply the business miles times the rate for that year. if you used a vehicle for one business say starting in 2019 but for the other in 2023 use 2019 for that schedule c and 2023 for the other
when you fill out worksheets for multiple vehicles for a business, there will be a supplemental report included which will shows the PART IV info for each vehicle which reports the in-service date
