According to Scotus, Father and Son are one spirator of the Holy Spirit (as I have just noted, 'spirator' functions as if it were a substance-sortal here) . . . Because there is only one substance here, Father and Son are just one spirator. Thus, the general rule is that, when we count substances, we do just that: we do not count supposita as such. (Of course, created natures always coincide with supposita, although the way in which we would define what it is to be a nature will be different from the way in which we would define what it is to be a suppositum, such that the difference is spelled out in terms of a distinction between indivisibility/individuality and incommunicability.) Contrariwise, the Father and Son are indeed two spirantes (spirating persons, persons who spirate) . . . According to Scotus, there are two divine persons spirating the Holy Spirit, and we can thus talk about two spirantes. However, according to Scotus, for example, 'two Gods' would refer to numerically two divine substances, and 'two human beings' to numerically two human substances. Factually, there are (at least) two human beings; so 'two human beings' has a genuine reference. Yet there is necessarily only one divine substance; so 'two Gods' can never refer to anything other than objects in a counterpossible state of affairs--it can never refer to anything in any actual or possible world. 'Person', of course (or at any rate, 'suppositum'), does not, in an Aristotelian universe, pick out a natural kind.
--Richard Cross, "Duns Scotus on Divine Substance and the Trinity," Medieval Philosophy and Theology 11 (2003), 194-195.
