For my husband.    I am planting no till soybeans in 30" rows with a a 12 row Kinze planter.    I am having trouble holding water hemp , late in 30"  rows.   I am considering double planting soybeans to get 15" rows.   Years ago I double planted with a 8 row 36" planter and it worked okay.    I would drive down and back on same wheel tracks with tractor draw bar offset  9".  If soil conditions were not good I would have trouble with emergence in wheel tracks.   
I am considering doing something with a 12/30" with offset of 7 1/2 inches.   I worry that the modest trouble I had with a 8/36" will be worse with a 12/30".   Covering the acres would not be a problem.    I am looking at a 6 row Kinze with a 5 row interplant.   I am running 2 planters now both 12/30".  One in corn and one in soybeans.   
I would like to hear from someone double planting on 30" rows.    Years ago,  this was more common.   I do not know of anyone doing this now.