I use one vehicle for two businesses, so it is used for business more than 50% of the time but not for any one business. How do I get TurboTax to recognize that?

Each business should report the correct percentage of use.  You need to enter your true total mileage on both businesses and report the miles driven specifically for each business on that respective Schedule C.  If the miles calculate to 30% for one and 35% for the other, then the combined totals for both your businesses is the 65%.  But, because neither business uses the vehicle over 50%, you cannot take a Section 179 expense or bonus depreciation.