Well most of the time the cables can be put on one way with all of the 20 pins. One drive becomes drive 1 and the other becomes drive 2. But sometimes they the connectors are made in such a way they can be put on either way.
The card need to go into slot 6 if I am not mistaken. A pr#6 will try to boot drive one. But it seems you are missing an 80 col/64k for a 1Ie 128k.
Take Care