As I see it, in Scripture, wherever a group of "24" are referred to, it is always just a solid group of 24... not "12 PLUS 12 [others]".
I see these (a "group of 24") in 1 Chronicles 24:7-19 (the "24 courses" of priests);
1 Chronicles 25:1-31 (the "24 divisions/courses" of musicians);
and Numbers 1:3-16[10:4] ("24 names put together" [re: the LEADERS / HEADS] for the numbering of the men of war).
See the following, that I've posted in past posts (for example of one of the above-mentioned):
[quoting from old posts]
[quoting Wm Kelly]
"In 1 Chronicles 25 we have the service of song. "Moreover, David and the captains of the host separated to the service of the sons of Asaph, and of Heman, and of Jeduthun, who should prophesy with harps, with psalteries, and with cymbals." It is called "prophesying" because it so directly brought in God, which is the emphatic meaning of prophesying. "And the number of the workmen according to their service was" - so and so. There were twenty-four courses of the singers. Now, this was another remarkable change. In the tabernacle, song was not the characteristic feature, but sacrifice; but in the temple in the day of glory, the song of triumph is the new and suitable feature."
--William Kelly, 1 Chronicles 25
[end quoting Wm Kelly; bold, color, and underline emphasis mine]
[recall my posts about Heb9:8-9a "which is a PARABLE for the PRESENT TIME" (referring to "the tabernacle in the wilderness" due to the "furnishings" named in v.4)... and involving the "STASIS / STASIN" word]
...see also the following commentary excerpt:
[quoting Gaebelein; at BibleHub under Commentaries]
"The Twenty-Four Divisions [Courses] of Musicians" - 1 Chronicles 25
"As we have seen before, Asaph [means "gatherer" or "removed"-TDW notes (from verb-form meaning "assembled, disappear, received, taken away, withdraw")], Jeduthun (Ethan) [means "praising" or "literally, to use (i.e. Hold out) the hand"-TDW notes] and Heman [means "faithful"-TDW notes] were the master leaders in song and music; their service was eminently spiritual, for we read “they should prophesy.” Heman especially is called the king’s seer in the words of God. This is a significant expression. How much there is in what is termed “worship”, which has nothing whatever of the words of God in it. In most of the songs used in our times there is little of the words of God and many contain unscriptural and sentimental phrases. Israel’s worship in song and music was to be spiritual, prophesying and in the words of God. Christian worship is not less. It is to be in spirit and in truth. “Let the Word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord” (Colossians 3:16). Asaph had four sons, Jeduthun six, and Heman fourteen, equal to twenty-four. They were divided into twenty-four courses of twelve men each, equal to 288, who served a week in turn. The names of six of the sons of Heman form, in the Hebrew, a complete sentence. Giddalti, Romamti-ezer, joshbekashah, Mallothi, Hothir and Mahazioth (verse 4) may be rendered in English:
I have magnified and I have raised up help;
Sitting in trouble, I have spoken oracles plentiful.
This fact has aroused the suspicion of the critics regarding the genuineness of this entire list of names. “Now this sentence,” saith a critic, “is either an obscure and ancient prayer which hath been mistaken for a list of names by the compiler, or else the compiler has purposely strung together those significant names in such order as to form a sentence” (W.R. Harvey-Jellic). But it is not the mistake of the compiler or an invention. We read that God gave to Heman these sons and the pious Israelite named his sons so as to produce this meaning. There are many such messages in names throughout the Bible. (See annotations on Genesis 5.)
--Gaebelein, on 1 Chronicles 25 [at BibleHub]
[end quoting; bold, underline, and brackets mine; parentheses original]
[see esp vv.1,6-7,31]
[note: v.1 "with harps [see Rev5], with psalteries [/'earthen vessels'], and with cymbals"...hmmm...]
[end quoting from old posts]