See the release notes:

v6.1.0

As with all of our dot releases, the 6.0 release broke some APIs, and the dot release added a number of new features. The big features for 6.1.0 include those for reverse-mapping support. First, we detect cycles in mapping classes to automatically preserve references.

Much larger however is unflattening. For reverse mapping, we can now unflatten into a richer model:

public class Order {  
  public decimal Total { get; set; }
  public Customer Customer { get; set; } 
}
public class Customer {  
  public string Name { get; set; }
}

We can flatten this into a DTO:

public class OrderDto {  
  public decimal Total { get; set; }
  public string CustomerName { get; set; }
}

We can map both directions, including unflattening:

Mapper.Initialize(cfg => {  
  cfg.CreateMap<Order, OrderDto>()
     .ReverseMap();
});

By calling ReverseMap, AutoMapper creates a reverse mapping configuration that includes unflattening:

var customer = new Customer {  
  Name = "Bob"
};
var order = new Order {  
  Customer = customer,
  Total = 15.8m
};

var orderDto = Mapper.Map<Order, OrderDto>(order);

orderDto.CustomerName = "Joe";

Mapper.Map(orderDto, order);

order.Customer.Name.ShouldEqual("Joe");  

Dogs and cats living together! We now have unflattening.

Enjoy!