עַד־שׁ֨וּב אַף־אָחִ֜יךָ מִמְּךָ֗ וְשָׁכַח֙ אֵ֣ת אֲשֶׁר־עָשִׂ֣יתָ לּ֔וֹ וְשָׁלַחְתִּ֖י וּלְקַחְתִּ֣יךָ מִשָּׁ֑ם לָמָ֥ה אֶשְׁכַּ֛ל גַּם־שְׁנֵיכֶ֖ם י֥וֹם אֶחָֽד׃
ביז דער כּעס פֿון דײַן ברודער װעט זיך אָפּקערן פֿון דיר, און ער װעט פֿאַרגעסן װאָס דו האָסט אים געטאָן; און איך װעל שיקן, און װעל דיך אָפּנעמען פֿון דאָרטן. נאָך װאָס זאָל איך אָנװערן אײַך בײדן אין אײן טאָג?
Rashi
למה אשכל means WHY SHOULD I BE BEREAVED משניכם OF BOTH OF YOU? — The one who buries his children is termed שכול bereaved: so we find in the case of Jacob (43:14), “If I am to be bereaved (שכלתי) let me be bereaved”.
Rashbam
גם שניכם, if he will kill you he in turn will be executed by the blood avengers.
Radak
עד שוב...גם שניכם, one to include the other. We find a similar construction of lumping two people together when they share a single common denominator [such as death in this verse. Ed.] in Genesis 43,8 גם אנחנו גם אתה, “both we and you,” where the wordגם introduces two entities instead of merely adding one to another, and Yehudah tells his father that sending Binyamin with him will result in both his father and the brothers and their families being spared death from famine. The rationale of אשכול שניכם יום אחד, is simply that the death of one of the brothers will inevitably be followed immediately by the death of the other.
Or HaChaim
צוטריטלעך נאָר פֿאַר פּרעמיום־מיטגלידערRabbeinu Bahya
צוטריטלעך נאָר פֿאַר פּרעמיום־מיטגלידערTur HaArokh
צוטריטלעך נאָר פֿאַר פּרעמיום־מיטגלידערRav Hirsch
צוטריטלעך נאָר פֿאַר פּרעמיום־מיטגלידערRashi
גם שניכם OF YOU BOTH — If he attacks you and you kill him his children will rise and kill you. The Holy Spirit poured itself forth upon her and she prophesied (had a presage) that they would both die on one day just as it is stated in the Chapter המקנה לאשתו (Sotah 13a).