>While I have never seen 2 queen hives,  I know others have,  how common,  and what circumstances?

I have seen it suggested that around 10% of hives commonly have 2 queens.  Not sure that is true, but we do see 2 queens quite often - perhaps because we select for A.m.m.  Marking queens obviously increases your chances of seeing this.

>Do they fight in a supercedure? 

We usually see queens superseding in the autumn and the two queens seem to co-exist amicably, often on the same comb and sometimes walking over one another.  The old queen seems to die during the winter, although we once found a hive with mother and daughter present on first inspection in the spring - but the old queen was not seen on subsequent inspections despite a thorough search for her.  I have  no idea how they die, but given that they are often four years old I would suggest 'natural causes'.

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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