It's a grammatical rule, it applies to some verses in the NT and it's commonly used by Trinitarians to prove the deity of Christ ...basically as the name suggests, it was brought forth by Granville Sharp in 1798...there's a good definition in Wallace's Greek Grammar Beyond the Basics...that's the standard book that student use to learn biblical grammar at intermediate level as far as i know, i'm still a beginner but i use it sometimes lol, coz it's a good book for reference...
"When the copulative kai (and) 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 [ho] ("ho" means "the"), 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..."
The footnote says, " Sharp, Remarks on the Uses of the Definitive Article"
So basically if you apply Sharp's rule on those verses, it proves that Jesus is "God"...
2 Peter 1:1 "righteousness of the God of us and savior Jesus Christ"
2 Thessalonians 1:12:"the grace of the God of us and Lord Jesus Christ"
Ephesians 5:5: "...in the kingdom of the Christ and God"
Titus 2:13: "of the great God and savior of us Christ Jesus"
I'm not done with my studies of it, i'll come back on this thread when i'm done...