Kinze made those New Idea planters like the one in the picture below (I owned it and sold it last year).  I think they just called them double frame planters (when I was selling mine there was a Kinze one on either Tractorhouse or USFarmer).  I wouldn't ever go back to having 15" rows with the units all on one bar, just for the simple fact that trash and rocks will get stuck between rows occasionally, which is pretty much never a problem if you stagger the units.

I think if you want an even # of rows, you're going to have to buy something and modify it to get there.

Good Luck,

Mike

(lh rr (Medium).JPG)

lh rr (Medium).JPG (123KB - 397 downloads)