This is the cable which connects the two Winchester discs to the Adaptec controller. Getting this together was in fact the hardest part of the whole Double Winchester enterprise.
The Adaptec board has two sets of pins to take the two 20-way cables from the drives, so all you need is two leads.
But for the 34-way connection, there is only one connector, and it is an edge-card connector rather than a set of pins. So I had to find a way of connecting a BBC double floppy disc drive data cable with an IDC connector at one end to the edge connector on the Adaptec board...