Sources For Shnayim Mikrah V'Echad Targum
A person should always complete his Torah portions with the congregation; scripture twice and translation once. Even [a verse like]: “עטרות ודיבון” (Numbers 32:3) [should be read in this manner]. For one who completes his Torah portions with the congregation is rewarded that his days and years are extended.
Rav Huna bar Yehuda said in the name of Rabbi Ami
Berachot 8a-b
Although one hears the whole Torah during the weekly congregational reading one is obligated to read for himself the respective parasha of each week –scripture twice and Targum once– so that he concludes the reading of the parshiot with the public. For a verse that does not have [translation in] Targum, scripture should be read three times.
Rambam
Mishneh Torah, Tefillah 13:25
Authorities differ as to when is the ideal time to read (study) the weekly reading. To facilitate the weekly completion a common method is to read one ‘aliyah (“going up”; portion read at the Shabbat reading for or by an individual) each day of the week. This was also the custom of the Gaon of Vilna.
The Chofetz Chaim - Rabbi Yisrael Meir Kagan
Mishnah Berurah 285:8
Ba’al HaTurim revealed an allusion to this precept in the words;
“ואלה שמות בני ישראל” which form an acronym for; אדם אשר לומד הסדר שנים מקרא ואחד תרגום בקול נעים ישיר יחיה שנים רבות ארוכים לעולם.
“ואלה שמות בני ישראל” which form an acronym for; אדם אשר לומד הסדר שנים מקרא ואחד תרגום בקול נעים ישיר יחיה שנים רבות ארוכים לעולם.
Ba’al HaTurim
Perushei Ba’al HaTurim, Feldheim ed., beginning of Shemot
Reviewing the weekly Torah section is to enable every person to be proficient in the Torah.
The Levush - Rabbi Mordechai Jaffe
Levush 268:1
Later halachic authorities allow one to substitute with English translations and/or commentaries.
Teshuvot VeHanhagot 3:98, Avne Yashpheh vol. 8 pg. 151
Opinions vary whether “…and Targum once” exclusively refers to Targum Onkelos or other translations and commentaries may be substituted if such texts aid an individual’s understanding of the Torah.
Tur and Bet Yosef
O.C. 285 citing Geonim and Semag
Even though we hear the Torah reading each week at the public reading, we are obligated to read each parasha twice and Targum once, even [a verse like] “עטרות ודיבון”.
Rav Yosef (Joseph) Karo
Shulhan ‘Aruch O.C. 285:1
Even a talmid hacham who is occupied with other subjects of Torah should not neglect completing his Torah portions with the congregation.
Sefer Ha’Ittim, no. 179, pg. 267, as cited by Aharonim; cf. Iggrot Moshe O.C. 5:17