…without bringing both relata to mind, then that is correct; there is no doubt that causality cannot be conceived unless both the cause and the effect are conceived. Thus, its presence in the intellect is conditioned upon conceiving the two correlated things together.
But if you mean that a relational property cannot exist in external reality unless both things are present, this is necessarily false and is rejected by the consensus of rational people. The very essence of the Lord, exalted be He, by itself entails all the attributes He possesses—whether affirmative or relational—and this is acknowledged alike by Muslims, philosophers, and humanity at large. His being the Creator, Lord, and Agent of the universe is precisely what they designate as “cause” and “efficient principle.”
There is, to be sure, disagreement about the nature of creation itself: is it a privative notion identical with the created entity, as most of the Muʿtazilites and Ashʿarites claim, or is it something additional to the created entity, as maintained by the overwhelming majority of Muslims—the position of the early generations and the leading scholars? Al-Bukhārī recorded that this is, without reservation, the view of the scholars and mentioned no dispute over it. A full discussion of the matter has been offered elsewhere.
The point relevant here is that on any of these opinions no rational person would say that the creature made Him a Creator, that the effect rendered the cause a cause, or that it is a condition for that. Rather, the Creator—glorified be He—alone brought every creature into existence. According to the majority, His being Creator is realised by His power and will; it is not confined to the creatures themselves. Indeed, the act of creating precedes the created entity in reality—so how could creation be caused by the created? Likewise, the causality of the cause is prior to the effect in reality—how, then, could the effect be a component of the causality of the cause?
His assertion that causality is a relational property and that a single thing is insufficient for any relational property to obtain, requiring instead the presence of both correlates, rests on a confusion produced by the vagueness of his claim that “a single thing is not enough for them to obtain.” What he says is correct with respect to their acquisition in knowledge and conception; but regarding their external existence, a single thing can be the cause of many relational properties. The cause is the cause of the effect and of its own causality alike, and both are established by the cause alone.
Fatherhood and sonship are likewise correlative attributes that necessarily follow from begetting; the act of procreation itself brings both correlatives into being.