The only way this can be done from what I can see is to explicitly name each API controller. Below I have a SomeApiController class.
routes.MapHttpRoute(
name: "SomeApiController",
routeTemplate: "SomeApi/{action}/{id}",
defaults: new { controller = "SomeApi", id = RouteParameter.Optional }
);
routes.MapRoute(
name: "Default",
url: "{controller}/{action}/{id}",
defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);