Tandem Agri-Fab discs?

This is just a thought. Could you get some heavy angle iron, and either weld it to the end of the weight tray so you can do the same on the old disc's tray, (see my "CAD" crayon assited doodle below) or if welding isn't an option, bolts? Just not sure how that would work without seeing both discs setting in place as you'd envision them. If one disc follows the other, not sure how you'd handle any sideways forces exerted on the set up during an engaged turning manuver. You'd have to lift it out of the dirt to make any kind of change in direction to avoid bending the rear disc off to one side. Unteresting project, let us know if you proceed and how you approach it. Good luck!!

Well you could try the old farmer's ways of using wood. Couple of 2x4s across the top with fillers to fill the steel U channels. Thru bolt them to the steel U channels of both and it would last for a while until you get a welder 😉
Sorry for the lousy drawing, but shows a 2x4 frame with a couple of blocks to fit into the steel U channels all bolted with carriage bolts. You'd need 2 of these one at each end of the U channel frames. A is 2x4 and B is a 2x4 filler for the space in the U channels. But again, any lateral forces might skew the whole thing unless you X braced it some how.

Still, an interesting project.