Another idea would be to crimp another IDC
connector onto the cable
alongside the connector that's already there (the same side of any 'IBM
twist, for example). You could pull the pin 34 contact out of the
connector before fitting it to the cable. And of course you could use a
header socket, not an edge conenctor, to plug directly into the 3.5" drivc.
I didn't suggest that because a fair number of people like to leave
the equipment in "original" state and one never knows when one is
about to suggest improving the Mona Lisa by adding some facial hair
to the picture.
In which case, buy a suitable conenctor to fit the controller (either a
header socket or an edge connector), connectors for the 2 drives (all
IDC types) and a length of 34 way iDC ribbon cable. Make up a new drive
cable, with the 'IBM twist' and pin 34 pulled on the 3.5" connector .
When you've finished using the 3.5" drice, put the original cable back in