Tit 2:13 and 2 Pet 1:1 in the KJV read as follows:
Looking for that blessed hope, and the glorious appearing of the great God and our Saviour, Jesus Christ. (Tit 2:13)
Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ. (2 Pet 1:1)
In these two texts, the KJV translators understood the underlying Greek to refer to two persons (God [the Father] and Jesus); however, after the KJV was translated, a grammatical rule was rediscovered by Granville Sharp which demonstrates that the KJV reading is in error, and based on the grammar (definite article + noun + the coordinating conjunction και + anarthrous noun) refers to the same person, not two. To put it a bit differently:
When the copulative και connects two nouns of the same case, [viz. nouns (either substantive or adjective, or participles) of personal description, respecting office, dignity, affinity, or connexion, and attributes, properties, or qualities, good or ill], if the article ὁ, or any of its cases, precedes the first of the said nouns or participles, and is not repeated before the second noun or participle, the latter always relates to the same person that is expressed or described by the first noun or participle: i.e. it denotes a farther description of the first-named person. (Granville Sharp, Remarks on the Uses of the Definitive Article, p.3 as cited by Daniel Wallace, Greek Grammar Beyond the Basics: An Exegetical Syntax of the New Testament [Grand Rapids, Mich.: Zondervan Publishing House, 1996], 271)
For instance, the Greek phrase from Tit 2:13 τοῦ μεγάλου θεοῦ καὶ σωτῆρος ἡμῶν ("our mighty God and our Saviour"), per this rule, predicates both θεος and σωτηρ of one person (Jesus), not two (this rule also applies to 2 Pet 1:1). Compare the KJV with the rendition of Tit 2:13 in the NRSV, for instance (emphasis added):
While we wait for the blessed hope and manifestation of the glory of our great God and Saviour, Jesus Christ.
This rule is much more nuanced than what I have presented (e.g., it does not apply when [1] both nouns are impersonal; [2] neither is plural; [3] neither is a proper name) so for a fuller discussion, see:
Daniel B. Wallace, Granville Sharp's Canon and Its Kin: Semantics and Significance (Peter Lang, 2008).
In his book on the Christology of the Pastoral Epistles, Andrew Y. Lau offers the following reasons why θεος in Tit 2:13 is used, not of the Father, but Jesus:
(1) The expression “θεος και σωτηρ” was a stereotyped formula common in first-century religious terminology and was used by both Diaspora and Palestinian Jews in reference to Yahweh, thus denoting one deity. (2) The most satisfactory explanation of the anathrous σωτηρος is that two coordinate nouns referring to the same person are customarily linked by a single article; σωτηρ ημων is generally articular in the PE (7x), anathrous only in 1T1.1 (where one person is clearly in view) and here in Tit. 2.13. The complex grammatical point suggests that “if Paul wished to speak unambiguously of two persons, he could have written either του μεγαλου θεου και Ιησου Χριστου σωτηρος ημων, or του μεγαλου θεου ημων και του σωτηρος Ιησου Χριστου . . . it must remain improbable that Paul would have acquiesced in a form of words that would naturally be depicting Jesus as ο μεγας θεος και σωτηρ ημων if in fact he believed that Jesus was in no sense θεος.” (3) The exceptional use of μεγας with θεος is better explained if θεος refers to Christ than if it signifies the Father. (4) The significant parallelism of 2.13 underlines that “the great God” is “the Saviour,” for the “blessed hope” is the “appearance of the glory”:
τὴν μακαρίαν ἐλπίδα καὶ ἐπιφάνειαν τῆς δόξης
τοῦ μεγάλου θεοῦ καὶ σωτῆρος ἡμῶν Ἰησοῦ Χριστοῦ
(Andrew Y. Lau, Manifest in the Flesh: The Epiphany Christology of the Pastoral Epistles [Tübingen: J.C.B. Mohr, 1996], 244)
(Andrew Y. Lau, Manifest in the Flesh: The Epiphany Christology of the Pastoral Epistles [Tübingen: J.C.B. Mohr, 1996], 244)