I'm at a bit of a loss. I don't know the identity of the addressee(s) of Jerome's Prologus Galeatus, but other scholars seem to know, but I can't find anyone who actually cites any evidence for their assertions, and scholars are divided on the identity of the addressee(s), into two basic camps, but they don't even seem to realize that there is this split opinion.
Okay, a little background. The Prologus Galeatus is what Jerome calls his Preface to Samuel and Kings, the preceding note he attached to his Latin translation of these Hebrew books. This is his most famous preface because it gives Jerome's list of the canonical books for the OT, and in it he calls the deuterocanonical literature "apocrypha." I myself have written on this preface a few times (see here and here), but I have never wondered before to whom Jerome dedicated the preface.
The problem is that the preface does not actually name anyone as the addressee, though it is clearly addressed to someone. Near the end of the preface Jerome says this: "But I ask you also, the handmaidens of Christ..." Jerome is addressing his audience, and he seems to think this consists of plural females, given the form of the word he uses: handmaidens (famulae). But, he doesn't name them.
Unless you're reading the Migne text, where he does name them: Sed et vos famulas Christi (Paula et Eustochium), rogo ... (PL 29.604a). But there is no manuscript support for this insertion (according to the Roman edition of the Vulgate).
On the other hand, just a little earlier in the preface, Jerome addressed the reader in the singular: obsecro te, lector. So, he possibly gives divergent hints about the identity of his addressee(s).
The old NPNF translation says that the addressees are Paula and Eustochium, friends and patrons of Jerome. This makes a lot of sense; Jerome dedicated many of his works to this mother-daughter pair. This same view is found in the work of Alfons Fürst (here, p. 87), and in an article by Igino Cecchetti (in this Festschrift, p. 81), and in an article by L. H. Cottineau (in this book, p. 56). Again, none of these scholars cite any evidence for this view; they seem to assume that the names Paula and Eustochium are actually in the text. They're not.
Other scholars think that Domnio and Rogatianus were the addressees, or maybe just Domnio. Even though Jerome did dedicate some works to these men (see, e.g., the Preface to Ezra), this suggestion makes less sense to me because of the feminine form of famulae, mentioned earlier. At any rate, for this identification, see Pierre Nautin (TRE 15, p. 310: only Domnio); Christoph Markschies (here, p. 150 n. 117: only Domnio); Megan Hale Williams (here, p. 283: Domnio and Rogatianus). The reasoning here seems to be that at Ep. 48.4 (Vallarsi numbering: 49.4), Jerome says that he sent a copy of this translation to Domnio.
So, which is it? I don't know. I surely wish some scholars would cite some evidence for their assertions. If you have any insight, dear reader, I'd love to hear it.