Of course, for anything to do with Trinitarian theology Gregory of Nyssa, as one of the Cappadocian Fathers, will have something important to say. Whilst he doesn't address the issue of a split in the Trinity with regards to Jesus on the cross, he does address the suggestion of a split, albeit under a different guise. Gregory in On the Holy Spirit is addressing the suggestion from Macedonius that the Holy Spirit was just "a divine energy diffused throughout the universe: and not a person distinct from the Father and the Son." (Gregory of Nyssa, On the Holy Spirit, in NPNFV (New York: Christian Literature Publishing Co, 1892, 315-325), 315, n1). It's clear that Gregory disagrees with this suggestion, but whenever we disagree with someone else's teaching the question always arises "do I or don't I seek to address this? Is this a significant problem? Is this a hill I should die on?" For Gregory the answer is clearly "yes".
There is a danger lest through our silence error may prevail over the truth, and so the rotting sore of this heresy may invade it, and make havoc of the sound word of faith. (ibid, 315).
So how does Gregory address this suggestion? Well, firstly he recounts his position, which we would consider to be the orthodox view of the Trinity.
We.. confess that the Holy Spirit is of the same rank as the Father and the Son, so that there is no difference between them in anything, to be thought or named, that devotion can ascribe to a Divine nature. We confess that, save His being contemplated as with peculiar attributes in regard of Person, the Holy Spirit is indeed from God, and of the Christ, according to Scripture, but that, while not to be confounded with the Father in being never originated, nor with the Son in being the Only-begotten, and while to be regarded separately in certain distinctive properties, He has in all else... an exact identity with them. (ibid, 315-316)Next Gregory addresses the suggestion being made, which is similar to the one made recently at Asbury; that there was a period in time when the Trinity was somehow "split". At Asbury, it was on the cross; for Gregory it was at creation. The suggestion made, which Gregory refutes strongly, is that the Holy Spirit was not present or active during creation. It's clear that Gregory disagrees with this suggestion, but also there is some evidence of reluctance at having to deal with it at all. "The blasphemy of these theorists, which we have had to follow out in every step it takes, has caused us unwittingly to soil our discussion with the mud of their own imaginings" (ibid, 319 - he does have a way with words!). Here's Gregory's response to the suggestion above
We are not to think of the Father as ever parted from the Son, not to look for the Son as separate from the Holy Spirit. As it is impossible to mount to the Father, unless our thoughts are exalted thither through the Son, so it is impossible also to say that Jesus is Lord except by the Holy Spirit. Therefore, Father, Son, and Holy Spirit are to be known only in a perfect Trinity, in closest consequence and union with each other, before all creation, before all ages, before anything whatever of which we can form an idea. The Father is always Father, and in Him the Son, and with the Son the Holy Spirit. (ibid, 319)Given the indivisible unity of the Trinity Gregory asks of his opponents a fairly straightforward question "If [the Holy Spirit] was not present [at creation], they must tell us where He was; and whether, while God embraces all things, they can imagine any separate standing-place for the Spirit" (ibid, 319).
It is clear that, for Gregory, the Trinity was active in creation, not just God the Father;
The fountain of power is the Father, and the power of the Father is the Son, and the spirit of that power is the Holy Spirit; and Creation entirely, in all its visible and spiritual extent, is the finished work of that Divine power... we should be justified in calling all that Nature which came into existence by creation a movement of Will, an impulse of Design, a transmission of Power, beginning from the Father, advancing through the Son, and completed in the Holy Spirit. (ibid, 320).Returning to the suggestion that the Trinity was somehow "split" at the cross, it is clear that Gregory of Nyssa would have no part of this. Nor should we. Any suggestion of a split in the Trinity displays a fundamental misunderstanding of what the Trinity actually is. The Trinity cannot be split because it is, in essence, a Tri-Unity. If a split were to occur, then that unity is destroyed, even if only briefly, and we are left with tri-theism (worshipping three gods) not monotheism (worshipping one God). In the words of the Athanasian Creed; "We worship One God in Trinity and Trinity in Unity; neither confounding the Persons nor dividing the substance... The Godhead of the Father, of the Son and of the Holy Spirit, is One, the Glory equal, and the Majesty co-eternal."