Double Head Line indicates dual mentality, talented, concentration issues and fickle mindedness.
In my practice I found that if subject has Double Head Line on both hands then the subject may not be able to concentrate in one field. In case of Double Head Line strong Head Line needs to be considered as Head Line.
Some Indian Palmists consider double headline indicating remaining unmarried, problem in married life, famous person and famous businessman. (But this is not very clear as this is not true in every case).