Automapper - Multi object source and one destination

mapper.MergeInto<PersonCar>(person, car)

with the accepted answer as extension-methods, simple and general version:

public static TResult MergeInto<TResult>(this IMapper mapper, object item1, object item2)
{
    return mapper.Map(item2, mapper.Map<TResult>(item1));
}

public static TResult MergeInto<TResult>(this IMapper mapper, params object[] objects)
{
    var res = mapper.Map<TResult>(objects.First());
    return objects.Skip(1).Aggregate(res, (r, obj) => mapper.Map(obj, r));
}

after configuring mapping for each input-type:

IMapper mapper = new MapperConfiguration(cfg =>
{
    cfg.CreateMap<Person, PersonCar>();
    cfg.CreateMap<Car, PersonCar>();
}).CreateMapper();

Map has an overload that takes a source and destination object:

d = AutoMapper.Mapper.Map<sourceone, destination>(sourceone);

/* Pass the created destination to the second map call: */
AutoMapper.Mapper.Map<sourcetwo, destination>(sourcetwo, d);