What is this NATed device?
From the diagram here it seems that you terminate the PPPoE of the DSLs to the ZS. Then whatever is connected behind the ZS box will be using your private address space, including the NATed device. Once you have configured the load balancing of the DSL lines there is nothing else you need to do. Everything will be balanced between the 2 lines with a round-robin algorithm with the ratio that you will define as a weight per line. The balancing occurs per connection so it doesn’t really matter if you have one IP address or multiple.