Which of these forms is correct?
- X will be used both for Y and Z
- X will be used for both Y and Z
- X will be used both for Y and for Z
As NOAD say: "When both is used in construction with and, the structures following 'both' and 'and' should be symmetrical in well-formed English."
In the first example, the symmetry or parallelism of 'for Y' and 'for Z' has been lost.
The second and the third example are well-formed English.
All three are acceptable and in use. The form that is selected might vary, depending on what X, Y, and Z are, how the sentence is punctuated, what verb is in the sentence, and what preposition is used (other prepositions could be used, besides for, such as to, by, or on).