Matthew Poole and continuators on 1 Samuel 18:21

21 And Saul said, I will give him her, that she may be a snare to him, and that the hand of the Philistines may be against him. Wherefore Saul said to David, Thou shalt this day be my Son in law, in the one of the twain .

i. e. Suddenly, within a time which probably Saul prefixed.

Whereas I have onely two Daughters, and thou wast disapointed of thy expectation in the one by an unexpected accident, thou shalt certainly have the other, which is the same thing. Heb. in the twain. Thus the Cities of Gilead is put for one of them, Iudg. 12. 7. And the sides of the Ship for one of the sides, Ionah 1. 5. Or, he saith, in the twain, or, in both, because he was in effect Betrothed to the one, and should be Married to the other, and so was his Son in law upon a double account.

Marginal references: Verse 26.