Here's my view, on how this may need to be setup, on a 12 row. (assuming only one seed transmission)

First, you will have to use JD  108 hole soybean disc (roughly uses 8.0 -8.5" of vacuum) and use JD 30 hole corn disc (uses roughly 8.0 -8.5" of vacuum)

Next, planter will be set up to plant soybeans on each set of outside 3 rows. = rows 1,2,3, and rows 14,15, 16. (Thus center rows, will be corn) [But could be swapped, but guess rows would be in the corn rows, then.]

Now, the ratio between plate holes, is 3.6 , so it we set up planter seed transmission  at a normal 44,000 corn population, then soybeans population will at 158,400.

So, now we are at the point that we need to modify gear sprockets.(for certain corn rows)

 We need to under drive, for one population and overdrive for the other.  44,000 x .75 (ratio) = 33,000 population and then 44,000 x 1.25 (ratio) = 55,000 population

This may need to be a combination, of working on both that 28 tooth sprocket OR maybe might have to alter the 19 tooth drive sprocket (at hex shaft) {No, I don't have that worked out, yet}

{Edit: I just see on another thread, you are looking for soybeans population at 118,800, so your plan is to have seed transmission at 33,000 corn setting. Thus overdrive, for both higher corn settings} This might be harder to do, because there is such a "big jump" from 33,000 to 55,000.