As a general term, the running of two films simultaneously in contact through a camera or printer aperture to expose both together; often used in special effects techniques.
When applied to a colour process, bi-pack denotes the location of two colour-sensitive emulsions on separate films which are exposed simultaneously. Now obsolete 1.
Also refers to 2-colour process release prints which have emulsion (dye) on both sides. Characteristically the colours of the dyes will depend on the manufacturer. Some are red and blue, others are green and brown.
1 1978, Focal Encyclopedia of Photography, Desk Edition Reprint, Focal Press, London, New York