I once had a 2 row planter, too.
I wanted to plant the permimeter of our 200 acre farm with SUNFLOWER SEEDS, here in PA.
The only problem was having the proper SEED varity discs in the seed hopper.
Finally I got the report that those discs were no longer available in the complete set of factory supplied discs. Corn, cotton, soybeans, etc.
So, I gave up and scapped the whole idea of trying to create one of those GOLDEN fence rows like I saw over the midwestern states, 50 yrs. ago.
Hang in there. just keep going with your own engineering and make what YOU need to do.
Try PLANTERS.com Get the picture???