St. Irenaeus of Lyons

Theological Format

St. Irenaeus of Lyons make it clear that something cannot be in two substances, but rather, these things must always be One Substance because God is One Substance. He writes thus in Against Heresies:

"All things therefore are of one and the same substance, that is, from one and the same God; as also the Lord says to the disciples 'Therefore every scribe, which is instructed unto the kingdom of heaven, is like a man that is an householder, which brings forth his treasure things new and old.' (Matthew 13:52) He did not teach that he who brought forth the old was one, and he that brought forth the new another; but that they were one and the same."

[St. Irenaeus of Lyons; Against Heresies, Book IV, Caput IX, Par. I]

Patristic Support

Now laying that out, if Irenaeus is to be consistent in his theology then he is forced to affirm Miaphysitism. Otherwise if he says Christ is formed of two different substance or is in two different substances then his theology fails; which he recognizes in the following:

"The Gospel, therefore, knew no other son of man but Him who was of Mary, who also suffered; and no Christ who flew away from Jesus before the passion; but Him who was born it knew as Jesus Christ the Son of God, and that this same suffered and rose again, as John, the disciple of the Lord, verifies, saying: 'But these are written, that you might believe that Jesus is the Christ, the Son of God, and that believing you might have eternal life in His name' (John 20:31) - foreseeing these blasphemous systems which divide the Lord, as far as lies in their power, saying that He was formed of two different substances."

[St. Irenaeus of Lyons; Against Heresies, Book III, Caput XVI, Par. V]

and again: 

"For just as the wood, which is the lighter body, was submerged in water; but the iron, the heavier one, floated: so, the Word of God's oneness with the flesh, is a becoming of one (ἑνωθἑντσς) according to hypostasis and nature, the heavy and terrestrial, having been rendered immortal."

[St. Irenaeus of Lyons; Fragments from the Lost Writings of Irenaeus, 28]