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The Chalcedonian terminology is 'in two natures' and not 'out of two natures'.
Indeed 'out of two natures' was rejected at Chalcedon. The issue is not the Christology of the Chalcedonians as it has been mediated through especially the council of Constantinople 553, but what we think of Chalcedon as an event itself. This is what is problematic.
There are issues around anathemas and the use of terminology in different ways. But these are clearly surmountable since the Christology is the same. Much of the problem has been with misunderstanding and misrepresenting other's views.
I think it is mistaken to say that St Severus does not use the term 'out of two natures'. I would say that it is foundational for him and for all non-Chalcedonians since it describes the union as being of two distinct elements.
So we do find it jarring when Leo of Rome says that the Word receives glory while the flesh receives suffering because we want to say that if the Word does not also receive suffering then WHO does! But to find deficiency in the Tome of Leo is not the same as finding deficiency in the Christology of those who receive the Tome since it may be understood, as all human communication may be understood, in a variety of ways.
Have you read my articles on the Christology of St Severus and other important Fathers? They are available in the book and ebook 'Orthodox Christology' which you, or anyone else, can purchase here...