This is what the manual says. I haven’t experimented myself with this but seems to explain it quite well 
“The secondary 2-operator layer in TWISTfm features Operators 5 and 6. These operators can be arranged either in series or parallel. In series mode, Operator 6 produces the sound, and Operator 5 modulates it for added complexity. In parallel mode, both operators function independently, akin to stacked oscillators in a traditional synthesizer, allowing for straightforward, layered sound generation.”