Here are the mass chants for this feast, Ss. Simonis et Iudæ, Apostolorum, at the website of the Benedictines of Brazil.
Here is the gradual, "Constitues eos (mp3)"; here's the chant score:
That comes from Psalm 45, verses 17 - 18: "Instead of thy fathers thou shalt have children : whom thou mayest make princes in all lands. I will remember thy Name from one generation to another : therefore shall the people give thanks unto thee, world without end."
The Introit, Iudicant sancti gentes, seems to be taken from Wisdom 3:8: "They shall judge nations and rule over peoples, and the LORD shall be their King forever." Here's the image of an illuminated manuscript; the last words on the page are Iudicant sancti gentes; the image comes from this interesting site, the Medieval Manuscripts section of "The Free Library of Philadelphia":
The Hebrew Bible reading for the day is from Deuteronomy 32:1-4:
Moses recited the words of this song:
Give ear, O heavens, and I will speak;
let the earth hear the words of my mouth.
May my teaching drop like the rain,
my speech condense like the dew;
like gentle rain on grass,
like showers on new growth.
For I will proclaim the name of the LORD;
ascribe greatness to our God!
The Rock, his work is perfect,
and all his ways are just.
A faithful God, without deceit,
just and upright is he.
Here's a gorgeous icon of these two (relatively unknown) Apostles: