Thanks for all of the comments. Yeah, I kind of suspected that I messed up something that was at least functioning. I'd been thinking of going to a Shimergo setup, but of course that takes a little investment. I'll have to say that the shifting is now very crisp, aside from the lever sticking issue. This is on a Santana tandem, and the cable run to the RD is pretty long. It goes through a couple of guides on the bottom tube along the way. There might be some friction there.

Edit: I now have no doubt there's extra friction going through the guides. I need to find some very small-diameter cable housings to feed through the holes. One cable already has one through one guide. So I need three more. Any suggestions?