Two lines penned by each girl (4)
I believe the answer is:
ella
'girl' is the definition.
(girl's name)
'two lines penned by each' is the wordplay.
'two lines' becomes 'll' (two abbreviations for line).
'penned by' indicates putting letters inside (inserted letters are penned in).
'each' becomes 'ea' (abbreviation).
'll' placed within 'ea' is 'ELLA'.
(Other definitions for ella that I've seen before include "Girl" , "Miss" , "Great late jazz singer . . . . Fitzgerald" , "Ms Fitzgerald" , "Woman" .)