Deductions & credits

The tax liability is based on your joint tax return, not individually.   And your tax lability is not based on the taxes withheld from your income.  It is the total tax liability on your tax return Form 1040 Line 18, based on your taxable income on the Form 1040 Line 15.

You can purchase as many vehicles as you want as long as you or your spouse are the registered owners of the vehicles purchased.