Via https://bitcointalk.org/index.php?topic=1242247.0
var pubkey = ExtPubKey.Parse("xpub6CUGRUonZSQ4TWtTMmzXdrXDtypWKiKrhko4egpiMZbpiaQL2jkwSB1icqYh2cfDfVxdx4df189oLKnC5fSwqPfgyP3hooxujYzAu3fDVmz");
var newAddress = pubkey.Derive(0).Derive(0).PubKey.GetAddress(Network.Main);
Console.WriteLine(newAddress);
It works and gives expected results.