Another clear solution, you can use constants to keep convention and add multiple [Authorize] attributes. Check this out:
public static class RolesConvention
{
public const string Administrator = "Administrator";
public const string Guest = "Guest";
}
Then in the controller:
[Authorize(Roles = RolesConvention.Administrator )]
[Authorize(Roles = RolesConvention.Guest)]
[Produces("application/json")]
[Route("api/[controller]")]
public class MyController : Controller