One bone between two people (7)

I believe the answer is:

patella

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'one bone' is the definition.
(patella is a kind of bone)

I cannot understand how the remainder of the clue works.

(Other definitions for patella that I've seen before include "bit of body usually under trousers!" , "Knee bone" , "Kneecap (anatomy)" , "positioned on each knee" , "showing a bit of leg" .)

I've seen this clue in the Evening Standard.

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