James Tabor devotes a page on his website to the Corinthians Correspondence  which emphasizes the concept of 2 Corinthians being composed from four distinct documents/letters and a free floating fragment. While I have disagreement with some the specifics, I do find agreement in the labeling of the segment from 6:14-7:1 as "floating" in the Catholic version handed down to us, as clearly 6:11-13 should be joined with 7:2-4. But the Tertullian and Dialogue Adamantius clearly show that at least portions of verses 6:14 and 7:1 were in Marcion's version of 2 Corinthians, while there is no attestation of the text surrounding this "floating" fragment.
Examining the entire text of 6:14-15 it can be broken into five phrases:
Μὴ γίνεσθε ἑτεροζυγοῦντες ἀπίστοις· Do not be mismatched with unbelievers;
τίς γὰρ μετοχὴ δικαιοσύνῃ καὶ ἀνομίᾳ, for what partnership has righteousness and lawlessness.
ἢ τίς κοινωνία φωτὶ πρὸς σκότος; or what fellowship has light with darkness?
τίς δὲ συμφώνησις Χριστοῦ πρὸς Βελίαρ, but and what harmony of Christ with Belial
ἢ τίς μερὶς πιστῷ μετὰ ἀπίστου; or what part has a believer with an unbeliever?
The middle phrase "τίς κοινωνία φωτὶ πρὸς σκότος" is attested in Marcion and carries with it the duality of light and darkness as opposite forces. But it is the surrounding text that gives it context. The first phrase is a command "Μὴ γίνεσθε ἑτεροζυγοῦντες ἀπίστοις" prohibiting the readers (e.g., faithful Christians) not to be partnered with those who are unfaithful (ἀπίστοις), no doubt implying marriage among other dealings. Who these unfaithful are is spelled out in the next phrase, asking what partnership (μετοχὴ) have righteousness and lawlessness (δικαιοσύνῃ καὶ ἀνομίᾳ), that the unfaithful in question are those who do not follow that Law, that is the Mosaic Law. Who are these lawless unfaithful, whom the righteous need to warned about having dealings with, even marrying, and sharing Christian fellowship with?
Verse 6:15 provides the clear answer. First the question is asked in what harmony (συμφώνησις) is Christ with Belial (Βελίαρ), a strange term that harkens primarily to Masoretic text Deuteronomy 13:13 [13:14 LXX] and the men of Belial, which is often incorrectly translated as "wicked" (λοιμός = pestilent, as in 1 Samuel 1:16, 25:17, 25, and 2 Samuel 2:12) in our modern English texts. Before continuing some explanation is required to understand the text.
In the Masoretic text the sons of Belial (בני בליעל) was various translated to the sons of lawlessness "υἱός παράνομος" (Judges 19:22, 20:13, 1 Kings 21:10, 13, 2 Chronicles 13:7, 2 Samuel 20:1) or more literally "sons (men) who hold contrary the law" (see Acts 23:3), or as in our case as lawless men "ἄνδρες παράνομοι or ἀνήρ παράνομος" (Deuteronomy 13:14, 2 Samuel 16:7).
The text of 2 Corinthians 6:15 clearly maps in this case to Deuteronomy 13:13 of the Masoretic text, where the writer is aware of the translation בני בליעל to ἄνδρες παράνομοι by the LXX. The equation thus is that there is no harmony between Christ and that "one" contrary to the Law. And who that one is becomes clear reading the rest of Deuteronomy 13:13 [13:14LXX] where the men of Belial say 'Let us go and serve other gods whom you have not known' λέγω πορεύομαι καί λατρεύω θεός ἕτερος ὅς οὐ οἶδα (נלכה ונעבדה אלהים אחרים אשר לא ידעתם). So there it is, the author of verse 6:15 is not only familiar with Hebrew and Greek, he clearly sees Beliar as representing "θεός ἕτερος ὅς οὐ οἶδα" another God, unknown to you, the Marcionite (and Gnostic) God, not the Jewish God of the Law. So finally having made the association he concludes by asking what part can the faithful πιστῷ (i.e. orthodox Christian) have with an unfaithful ἀπίστου (i.e., heretical Christian), referring back to the opening phrase where righteousness is not associated with the unfaithful.
For completeness of the section, verse 6:16 makes clear there can be no compromise between this Christian God and idols, and then follows with quotes from Leviticus 16:12, Ezekiel 37:27, Isaiah 52:11, Ezekiel 20:3, and Samuel 7:8, 14, which buttress the incompatibility of the Christian/Jewish God and other practices. But salient is Isaiah 52:11 (verse 6:17) which refers to the faithful above with the Lord commanding them to separate themselves from the previously described unfaithful Belial, quotes Samuel that doing this he promises become a father to them.
So we see up through was necessarily written by the Catholic editor, as the promise referred to in 7:1(a) is stated in verse 6:17-18.
It has become clear that only τίς κοινωνία φωτὶ πρὸς σκότος was present in Marcion's text, the rest of 6:14-7:1(a) was written by a later Catholic editor.