Anything you can do to index (locate) the fertilizer more precisely to the seed and below the surface (not necessarily 8" maybe 2-4"for beans especially) and covered will help you make more efficient use of your fertilizer and cut the rate compared to broadcast for a similar crop response.

 On the twin rows - twins are usually started because of corn. Twin row corn (usually 2 rows 7-8" apart on 30" centers) allows you to have more uniform canopy and sunlight capture yet still be able to harvest with a 30" corn head. They are usually more beneficial in short cool season areas.

If you plant twin rows 10 or 12" apart you will have trouble harvesting corn with a single corn head row unit without substantial header losses - 12" is a long way to lean the corn in to the rolls compared to 7 or 8" especially if the ears are low on the plant.

If you plant both corn and beans, a twin row system with 2 rows 8" apart on 36" center will work and you can put the fertilizer between the twin rows. This will give you bean rows about 28" apart (36"-8").

If you are just planting beans with this planter why not go to 18" single rows for beans, run fertilizer coulters 18" apart in the fall and plant right over the top in the spring. Or run half the number of coulters and place the fertilizer, mostly p & k  down the middles in the fall. The roots will get there in the spring.

All depends on your rotation. Do you have cotton or milo in rotation? is cotton on 36" centers?

Jim at Dawn