Complex sentence: Japanese slaves were brought by the Portuguese to Macau, where some of them not only ended up being enslaved to Portuguese, but as slaves to other slaves, with the Portuguese owning Malay and African slaves, who in turn owned Japanese slaves of their own.
Split into simpler sentences: Japanese slaves were brought by the Portuguese to Macau, where some of them were enslaved to Portuguese, but could also be bought by other slaves. Thus, the Portuguese might own Malay and African slaves, who in turn possessed Japanese slaves of their own.