bar magnet question

[John Bull]

Can someone give please give me a satisfying explanation to the
following observation from one of my year 13 students.
 
If you put to bar magnets with like poles facing they repel. If you
separate them with a keeper (about 2 cm long ) they are both attracted
to it. ie. the attractive force to the keeper is greater than repulsive
force between the poles. 
 
What is going on with field lines here?
 
cheers
 
John Bull
St John's College
Hamilton
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