How do I pass 2 lists into Parallel.ForEach?

You are looking for Enumerable.Zip

If you're using .NET 4 (which you probably are) and you're trying to pair the first Person with the first Car etc, you can just use Zip:

List<Person> a = new List<Person>() { new Person(), new Person(), new Person() };
List<Car> b = new List<Car>() {} { new Car(), new Car(), new Car() };
var zipped = a.Zip(b, (person, car) => new { person, car });

Parallel.ForEach(zipped, pair => {
    Person person = pair.person;
    Car car = pair.car;
});