Verified answer

They have the same primary meaning (a seat that you sit on).

However, 席 has additional meanings that 座席 lacks.

席　can denote rank or position. 部長の席を狙っている。 (I'm after the director's chair) would be one example.

席 can also denote a kind of gathering. お酒の席 (osake no seki) and パーティーの席 (PARTY no seki) are used in a similar way to 宴会 ("enkai", a party).