Complex sentence: Traditionally he is seen as Akhenaten's co-regent and immediate successor and predecessor of Tutankhamun and is assumed to be a close, male relative of those two kings (either by blood or marriage).
Split into simpler sentences: Traditionally he is seen as Akhenaten's co-regent and immediate successor, and predecessor of Tutankhamun. He is assumed to be a close, male relative of those two kings (either by blood or marriage).