I’m in agreement with you that you can definitely use the standard mileage rate or the actual expenses to deduct more than one vehicle used for business. Here is a link to a page on the IRS website that summarizes the standard mileage deduction. It also says that you can deduct maximum of 4 vehicles using the standard mileage rate: https://www.irs.gov/newsroom/2017-standard-mileage-rates-for-business-and-medical-and-moving-announced
Be sure to check out our mileage tracker app guide to see how much easier it is to track your miles for business using an app: https://fitsmallbusiness.com/best-mileage-tracker-app/
Thanks for reading!