Or HaChaim on Exodus 28:9/אור החיים על שמות כ״ח:ט׳

1 ולקחת את שתי אבני שהם, "You are to take the two Shoham-stones, etc." The word את in this connection needs explaining. Perhaps the expression is to draw our attention to the previously mentioned Shoham-stones at the beginning of Parshat Terumah (25,7). The meaning of our verse then would be: "you shall take the two Shoham-stones which I have already commanded you to bring (as gifts)." This would account for the מתג, hyphen, between the words אבני and שהם, i.e. why only these two gemstones have been singled out here by name. These two stones performed a function additional to that of the gemstones called אבני מלואים, "stones to be set" (where the Torah did not employ the hyphen).