from Wiktionary, Creative Commons Attribution/Share-Alike License.
- noun cinematography The process of
loadingtwo reelsof filminto a camera, so that they both pass through the camera gate together, formerly used for visual effectsand as a subtractivecolour process.
from Wiktionary, Creative Commons Attribution/Share-Alike License
I believe you had to have two separate rolls of film to bipack the matte and BG - so that you could come back and lay in the characters on another run through the opt printer.
The other film in the bipack was only sensitive to the sodium vapour light, so that only the matte showed up on that film.