Sorry its a little late, but
This is the way I have decided to add duals to the 312 (will work on any tractor with the standard 12" rims). Works good, but I havent had a chance to push it yet, the Ags are on order to replace the turfs (one is flat anyway).
Shows the parts used, the steel ring (2x1/8'' flat bar) fits inside the first lip of the rim. the threaded rod holds the 2 rims together.
Shows the holes that the threaded rod goes through, nuts hold it together, 4 per side.
The scene inside the rim, shows the ring sitting in the rim, and the threaded rod
If I had another set of rims and tyres I would put on triples just for laughs