(i) When using both . . . and, he careful that these words are in their right positions and that each carries equal weight. Nothing that comes between the both and the and can be regarded as carried on after the and. If words are to be carried on after the and they must precede the both; if they do not precede the both they must be repeated after the and. For instance:
He was both deaf to argument and entreaty.
Since deaf to comes after both it cannot be "understood" again after and. We must adjust the balance in one of the following ways:
He was both deaf to argument and unmoved by entreaty.
He was deaf both to argument and to entreaty.
He was deaf to both argument and entreaty.
(ii) Do not use both where it is not necessary because the meaning of the sentence is no less plain if you leave it out:
Both of them are equally to blame. (They are equally to blame.)
Please ensure that both documents are fastened together. (. . . that the documents are fastened together.)