partial class MyTestService : ServiceBase
    {
...
private IList<ServiceHost> _hosts = new List<ServiceHost>();

       protected override void OnStart(string[] args)
        {
            _hosts.Add(new ServiceHost(typeof(DefaultCalculatorService)));
            _hosts.Add(new ServiceHost(typeof(SomeOtherService)));

            foreach (ServiceHost host in _hosts)
            {         
                host.Open();
            }
        }