Bimat is no more, and I doubt if there is anyone around that still knows the formula.

Basically it was a monobath in a thick coating on one film, and a regular film for taking the photo. There was no image transfer involved and the bimat "donor" was peeled away from the image layer which was scanned for transmission to earth. There was some sort of delaminating chemical that allowed for a clean separation between the two without adhesion of any of the chemistry to the film layer, but which allowed a clean lamination for uniform imaging.