Let's say the Crown of Winter is in play, and the Crown of Azor Ahai is in play, too.
Crown of Winter:
If the Black Raven card is not in play, it is Winter.
Crown of Azor Ahai:
If Crown of Azor Ahai is standing and the White Raven card is not in play, it is Summer.
Assuming the Crown of Azor Ahai is standing, and there are no other ravens in play, is it both Summer and Winter?