How can I preview each deck in MegaSeg with Hercules DJ Console MK2?

MegaSeg is a DJ mix software in Mac OS.

1) In MegaSeg, if you use the Discrete Output Mode, and select both Hercules outputs using the Aggregate device output, then you can use DJ Console MK2 headphones selector to select the various cue modes.

2) But MegaSeg default mode is different, since instead of the left and right decks being hardwired to one output, MegaSeg has a "Preview" button to route either deck to a 2nd output for cueing or previewing. In this mode, both decks would output to a single output device (on a single stereo channel), and then you can press Preview and switch either deck to a 2nd output to cue (like the built-in headphone jack). This function is mapped to the cue button on the DJ Console MK1, and now the Audio source select on the DJ Console MK2. As the audio source select is also on this button, it means you must push this button twice to toggle the Preview mode.