It seems the adjustment screw for the first curtain take-up drum spring was frozen to the drum. So, twisting the screw to change the tension merely twisted the drum.
this had the effect on the prior repair attempt. By twisting the screw the drum rotated, releasing any tension on the ribbons and the top ribbon got bunched up. After the frozen screw was loosened (with alcohol), and tension applied to the take-up drum, the ribbon became tight again, but it was twisted in two places. This prevented the second curtain from coming over.
This was a little of a challenge to fix without taking the curtains out. The upper ribbon passes tightly around rod #223 in the picture. As it passes around that rod, it is twisted and stays twisted.
Eventually I got the un-twisted portion to pass around the rod #223 and that fixed it.