אורח חיים קמ״אOrach Chayim 141

Se’if 1א׳

צָרִיךְ לִקְרוֹת מְעֻמָּד, וַאֲפִלּוּ לִסְמֹךְ עַצְמוֹ לְכֹתֶל אוֹ לְעַמּוּד, אָסוּר אֶלָּא אִם כֵּן הוּא בַּעַל בָּשָׂר. הַגָּה: וְכֵן הַחַזָּן הַקּוֹרֵא צָרִיךְ לַעֲמֹד עִם הַקּוֹרֵא (מָרְדְּכַי הֲלָכוֹת קְטַנּוֹת).

The Laws of The Reader and The One Who Reads [For Another], Containing 8 S'ifim One must read [from the Torah] while standing; and even to lean oneself on a wall or pillar is forbidden unless it is a heavy-set person. Gloss: And so too, the chazzan who calls up [the readers] must stand with the reader. (Mordekhai, Halakhot Ketanot)

Se’if 2ב׳

לֹא יִקְרְאוּ שְׁנַיִם, אֶלָּא הָעוֹלֶה קוֹרֵא וּשְׁלִיחַ צִבּוּר שׁוֹתֵק, אוֹ שְׁלִיחַ צִבּוּר קוֹרֵא וְהָעוֹלֶה לֹא יִקְרָא בְּקוֹל רָם; וּמִכָּל מָקוֹם צָרִיךְ הוּא לִקְרוֹת עִם הַשְּׁלִיחַ צִבּוּר, כְּדֵי שֶׁלֹּא תְּהֵא בִּרְכָּתוֹ לְבַטָּלָה, אֶלָּא שֶׁצָּרִיךְ לִקְרוֹת בְּנַחַת, שֶׁלֹּא יַשְׁמִיעַ לְאָזְנָיו. וַאֲפִלּוּ מַשְׁמִיעַ לְאָזְנָיו לֵיכָּא לְמֵיחַשׁ, דְּלֹא עָדִיף מִתְּפִלָּה, כְּדִלְעֵיל סִימָן ק"א (דַּעַת עַצְמוֹ).

Two [people] may not read [together], rather the one got called up should read and the prayer leader is quiet, or the prayer leader should read and the one who got called up does not read in a loud voice. In any case, one [who is called up] must read [aloud] with the prayer leader so that his blessing should not have been in vain, but rather should read along quietly such that it cannot be heard by one's own ears.Gloss: And even if one's own ears can hear, this is not a concern, for it is no different than [hearing himself while] praying [the Amidah] as was seen above in siman 101. [His own opinion].

Se’if 3ג׳

וְיֵשׁ נוֹהֲגִים לְהַעֲמִיד מִי שֶׁמְּקָרֵא לָעוֹלֶה מִלָּה בְּמִלָּה, וְאַחַר שֶׁגּוֹמֵר הַמְקָרֵא הַמִּלָּה אוֹמְרָהּ הָעוֹלֶה.

There are those who practice to have someone who reads word-for-word stand in [as an agent] for the person called up, [reading] word-by-word, and after the reader finishes each word, the person called up says it.