[Note: This is a mind dump of some thought I wrote down earlier this evening.]
The text of Eph 1.1 is all jumbled up! The NA28, UBS4, WH etc. all read:
τοῖς ἁγίοις τοῖς οὖσιν [ἐν Ἐφέσῳ] καὶ πιστοῖς ἐν Χριστῷ Ἰησοῦ
א2 A P and other add πασιν before the τοῖς ἁγίοιςp46 in all of it’s splendor, in my opinion, seems to make this less convoluted:
τοις αγιοις ουσιν και πιστοις εν χ̅ρ̅ω̅ ι̅η̅υ̅
While ἐν Ἐφέσῳ is absent in some early mss., it is strangely present in the Inscriptio (as can be seen in the official NLM graphic above). While titles were most likely added after the fact.
Either ἐν Ἐφέσῳ was not originally present, thus giving us the reading of p46, or Paul somehow got all tongue tied or something trying to add a destination. When you look at Col and Phil, Paul seems to have a much better grasp on the syntax:
Phil: πᾶσιν τοῖς ἁγίοις ἐν Χριστῷ Ἰησοῦ τοῖς οὖσιν ἐν Φιλίπποις
Col: τοῖς ἐν Κολοσσαῖς ἁγίοις καὶ πιστοῖς ἀδελφοῖς ἐν Χριστῷ
It is interesting to note the parallel passage in Col. There, Paul seems to make it clear that there are not two separate groups being addressed: saints and faithful ones. In Eph, this is not as clear. Also, the addition of πασιν in Eph seems to be a scribes way of trying to harmonize Eph and Col.
Also, an argument against the reading in p46 is that in Phil οὖσιν is specifically followed by a location; in Col, οὖσιν may be implied.
At any rate, I leave this open for your thoughts and further observations. There are more qualified text-critics among us, and it would be great to hear them weigh in and educate us on such matters like this.