Corrections: On the first setup, I said it was a "2-on-3", but I meant to say "3-on-2". I also forgot to mention that the main line will also need to be pulled through the Progress Capture (MPD) during the haul in order to prevent a dead-leg from forming in the main line. The ganged MA system in this case does not require any ratchet or progress capture device.
Ganging a separate rope with its own mechanical advantage system onto an existing tensioned working line is also ideal when the working line does not have enough length left in reserve to build a mechanical advantage, such as if the rope was only 200' long and 190' of it was deployed in service.