But [which] [for which] Bolinger (1978: 109) Incidentally, Bolinger notes that a certain restriction applies to the formation of more than one remnant wh-phrase.
the second remnant wh-phrase is required to be PP, not DP, which is shown by the contrast between grammatical (4) and ungrammatical (5): (5) *I know that in each instance one of the girls got something for one of the boys.