Two girls on one knee (7)
I believe the answer is:
patella
'one knee' is the definition.
(I've seen this before)
'two girls' is the wordplay.
'two girls' means two substitutions for 'girl'.
'girl' becomes 'pat' ().
'girl' becomes 'ella' (girl's name).
'pat'+'ella'='PATELLA'
'on' acts as a link.
(Other definitions for patella that I've seen before include "The formal kneecap" , "positioned on each knee" , "bit of body usually under trousers!" , "Kneecap (anatomy)" , "A pallet (anag.)" .)