the old planter used plates, that rotated, and picked up the kernels in a pocket and then ran them over a hole and dropped it down to the opener,
Corn-Planter-Plates | Life on a Colorado Farm
the URL above has a view of looking down in the seed canister,
the more modern version use fingers, or plates that have air or vacuum to hold the seed on the plate until it reaches the drop area, and some use more things to keep the seed perfectly spaced
but the old and simple was the plate that rotated and it was driven by a system of gears or chains and sprockets to drive off the wheels or there a drive wheel that lowers when the planter is lowered into the ground, (most use the transport wheels with a clutch system,
one could just hook two of the garden planters together, or use there metering device, to make a seeder,
I think there are a few of these type of units, the URL is for information only, not a recommendation,
http://www.earthway.com/product/garden-products/
one could consider, a "PLANET JR Planter" type unit as well, they are on ebay a lot, and a form of it I think is still made and sold,
http://www.coleplanter.com/coleplanetjr.htm