J, 30 29 25 bytes
1(|:(0{-.&,)"_1]\.)<\\.&>
<\\.&> a 3-dimensional array of substrings
1 |: transpose each matrix to sort the substrings by length
1 ]\. all choices where one word is
missing
(0{-.&,)"_1 for every matrix, flatten, remove substrings
that are present in the
corresponding complement,
corresponding complement, pick first