וְאֶ֨לֶף אִ֣ישׁ עִמּוֹ֮ מִבִּנְיָמִן֒ וְצִיבָ֗א נַ֚עַר בֵּ֣ית שָׁא֔וּל וַחֲמֵ֨שֶׁת עָשָׂ֥ר בָּנָ֛יו וְעֶשְׂרִ֥ים עֲבָדָ֖יו אִתּ֑וֹ וְצָלְח֥וּ הַיַּרְדֵּ֖ן לִפְנֵ֥י הַמֶּֽלֶךְ׃
And there were a thousand men of Benjamin with him, and Ziba the servant of the house of Saul, and his fifteen sons and his twenty servants with him. And they rushed into the Jordan before the king.
Siftei Chakhamim
Onkelos translates it וצלח [he split] as in וצלחו [they split]... It seems that Rashi is not proving from Onkelos [that ויבקע means “splitting”]. Rather, he is explaining that [also Onkelos’s translation of] צלח is “splitting,” as in (Shmuel II 19:18): “They split (וצלחו) the Jordan.”
Rashi
ויבקע AND HE CLEAVED [THE WOOD] — Its rendering in the Targum וצלח has the same meaning as the verb in (2 Samuel 19:18) And they rushed into the Jordan”, which signifies cleaving the waters; old French fendre; English to split.