you need to use the version with this signature
public static Route MapRoute(
this RouteCollection routes,
string name,
string url,
Object defaults,
string[] namespaces )
To make it work just set up two almost identical routes - one which includes the project1 for the namespaces parameter and then a second identical version but use project2 in the namespaces parameter. That said, it would generally be less confusing, to use different names if you can...
routes.MapRoute(
name: "Default_Project1",
url: "{controller}/{action}/{id}",
defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional,
namespaces: new string[] { "Project1" } }
);
routes.MapRoute(
name: "Default_Project2",
url: "{controller}/{action}/{id}",
defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional,
namespaces: new string[] { "Project2" }
);