Two girls, one on each knee (7)
I believe the answer is:
patella
'one on each knee' is the definition.
(kneecap)
'two girls' is the wordplay.
'two girls' means two replacements for 'girl'.
'girl' becomes 'pat' ().
'girl' becomes 'ella' (girl's name).
'pat'+'ella'='PATELLA'
(Other definitions for patella that I've seen before include "showing a bit of leg" , "Knee bone" , "A pallet (anag.)" , "Joint cover" , "Kneecap (anatomy)" .)