First, prepositions will sometimes have other prepositional phrases as complements:
(10) The plane emerged from behind the cloud.
From and behind are both prepositions. But notice that from behind the cloud forms a single constituent. You can move it to the front of a cleft sentence:
(10a) From behind the cloud is where the plane emerged.
(2) Ken looked up her number.
(3) Ken looked up her dress.
A little scrutiny will show that up does not have the same function in both sentences. For example, while we can create a cleft sentence with up her dress, we can't do the same thing with up her number:
(2a) *Up her number is what Ken looked.
(3a) Up her dress is where Ken looked.
Also, we can move up to the end of the first sentence, but not the second:
(2b) Ken looked her number up.
(3b) *Ken looked her dress up.