1st it is because it is alkhestry and not alchemy. Father only blocked regular alchemy. he probably could block Alkhestry but didnt think anyone else there would be using it or he just didnt know about it, but could probably block it if he wanted. And 2nd i think that both sages were Hohenhiem. Father definitely didn't go and spread alchemy or alkhestry for anyones benefit, and both sages are said to have helped people with their "miracles"(alchemy/alkhestry), so i believe that since Hohenhiem is so old that he visited both lands teaching each a form of Alchemy that was pertinent to that land. the more medical alchemy called alkhestry to Xing and the more military alchemy that was taught to Ametris

Remember Mr. Han, that guy that was guiding everyone towards the ruins of Xerxes, remeber he said there was a man. One was known as the "Sage of the East" that came to one country (Amestra or whatever). But then there was another one that went to Xing, he was known as the "Sage of the West" or something. I think the Sage of the West taught people "Alkhestry" and that "Father" = "The Sage of the East". He is wearing the same robes etc as the murals in Xerxes also. Let's see what happens.

There was that whole discussion of Alchemy vs. Alkehesty or whatever they called it like ten episodes back when they were in Xerxes. My guess is that the guy blocked Alchemy but didnt know enough about the other type to block it.