ASP.Net MVC Web api - MapHttpRoute for multiple controllers won't take more than 1 parameter

I might have misunderstood the concept of MapHttpRoute entirely. I'm developing an asp.net web api, having hard time getting the Routing to work.

I have a MyUserController.cs and MyTaskController.cs. MyUserController accepts username/password and returns a sessionkey, it works as expected.

But MyTaskController keeps complaining when I pass the second parameter. http://localhost:59720/api/mytask/somesessionkey - works

http://localhost:59720/api/mytask/somesessionkey/1 - doesn't work, throws the following error

{"Message":"The requested resource does not support http method 'GET'."}

I tried with and without [HttpGet] on the methods.

MyTaskController.cs

//works fine
public IEnumerable<MyTask> Get(string sessionkey)
{
    MyTaskModel mtm = new MyTaskModel();
    return mtm.GetAll();
}

//doesn't work
public string Get(string sessionkey, int id)
{
    if(isValid(sessionkey))
    {
        return DataAccess().GetTask(id);
    }
    return "";
}

MyUserController.cs

public string Get(string username, string password)
{
    string sessionkey = "tempsession";
    return sessionkey;
}

WebApiConfig.cs

public static void Register(HttpConfiguration config)
{
    // Web API configuration and services
    // Configure Web API to use only bearer token authentication.
    config.SuppressDefaultHostAuthentication();
    config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

    // Web API routes
    config.MapHttpAttributeRoutes();

    config.Routes.MapHttpRoute(
        name: "MyUserApi",
        routeTemplate: "api/{controller}/{username}/{password}",
        defaults: new { controller = "MyUser" }
    );

    config.Routes.MapHttpRoute(
        name: "MyTaskApi",
        routeTemplate: "api/Mytask/{sessionkey}/{id}",
        defaults: new
        {
            controller = "MyTask",
            id= RouteParameter.Optional
        }
    );

    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );

    config.Formatters.Remove(config.Formatters.XmlFormatter);
    config.Formatters.JsonFormatter.SupportedMediaTypes.Add(new System.Net.Http.Headers.MediaTypeHeaderValue("application/json"));
}