routes.MapRoute(
    "Id + Slug",          // Route name
    "Test/{id}/{slug}",   // URL with parameters
    new                   // Parameter defaults
    {
        controller = "Test",
        action = "Details",
        id = "",
        slug = ""
    },
    new { slug = new SlugConstraint() }
);

public class SlugConstraint : IRouteConstraint
{
    public bool Match(HttpContextBase httpContext,
        Route route,
        string parameterName,
        RouteValueDictionary values,
        RouteDirection routeDirection)
    {
        string value = values[parameterName].ToString();

        return value.Contains("-");
    }
}