I am using 2013.

On mine, "Ghost" doesn't seem to do anything at all unless a specific channel is selected,
ie, "All" is not selected as the channel.

With a specific channel selected, the "ghosted" notes will appear when one of the instruments
is selected with the B, D, P, G, etc buttons. Selecting one of these instruments also changes
the channel number to the channel used for that instrument. Then a new channel number can be
selected, leaving the "ghosted" notes for the previously selected instrument on the display.

I am guessing the purpose is to edit a new track while viewing an existing instrument track.
It doesn't seem to allow two instruments at once. Only one instrument for "ghost" notes, and
one track for editing.
(not sure if I am using the term "track" correctly here)

Anyway, that's all I can figure out. Also I found this under "Help":

"- Ghost display. When viewing a single channel, notes on other channels can be ghost-displayed in light gray."

-