Blass-Debrunner-Funk Greek Grammar (BDF §276.3) says that σωτῆρος is definite at 2 Peter 1:1, and so “σωτῆρος ἡμ. ᾽Ι. Χρ. may be taken by itself and separated from the preceding.”
BDF §276 (3) Cf. 2 P 1:1 (but here S has κυρίου for θεοῦ, probably correctly; cf. 11, 2: 20, 3: 2, 18)...