Like Dave said 20" rows can work quite well in 30" stalks. Few less rows to maintain and more room for residue than 15".

White planters with a splitter would be easy enough to add another unit on the back bar. Just need to add a little bar, get another unit and you're in business. Usually if the back bar has an odd number of rows there is an unused monitor wire on the harness.

 Everything (except the hitch on 6000 and 8000's) is bolted onto the bar on a White planter you can move rows and wheels to get the configuration you want. 

The 1st link below would work really nice set up as 12 row. 

http://www.tractorhouse.com/listings/detail.aspx?OHID=5552837&GUID=83AD1074263143A79236C9D0D553654C

http://www.tractorhouse.com/listings/detail.aspx?OHID=5581922&GUID=83AD1074263143A79236C9D0D553654C