Service VSPackage如何使服务在同一接口上实现？

我有两个包提供相同的服务SService1，当得到服务SService1时，会得到哪一个

public interface IService1 { ... }
public interface SService1 { ... }

[ProvideService(typeof(SService1))]
public sealed class Package1 : Package
{
public Package1 ()
  {
    var serviceContainer = this as IServiceContainer;
    var servicse1= new Service1();//implement SService1
    serviceContainer.AddService(typeof(SService1), servicse1, true);
  }
}

[ProvideService(typeof(SService1 ))]
public sealed class Package2 : Package
{
public Package2 ()
  {
    var serviceContainer = this as IServiceContainer;
    var servicse2= new Service2();//implement SService1
    serviceContainer.AddService(typeof(SService1), servicse2, true);
  }
}