[sanddrag]

Since you mentioned simplicity, I wont go into the welding options. If you have access to a mill and either a skilled operator or DRO then what you could do is drill four very precise holes in the sprocket that rides on the shaft and tap them. It is Very important that the bolt circle that these holes lie on is concentric with the bore. Then, make a spacer with a similar hole pattern but make them through holes just barely large enough to clear the screw. Then on your outboard sprocket, do the same thing. Bolt it together (and tight) and now you have a double sprocket to the spacing of your liking and you need not do anything to the shaft. The outboard sprocket is drven by the screws going through it. Remember to make sure you select a bolt hole circle such that the heads of the bolts do not interfere with the chain. At least that's what I would do, given the design considerations.

EDIT: A way to do this without machine tools would be to clamp both sprockets and the spacer together with a bolt through the bores (and a nut on the other side). Get everything lined up and then tighten up that bolt real good. Make white paint line down all three pieces so you can line them up easily later. Then set up this assembly on a drill press and drill through all three at once with the bit that the tap requires. Once you have the four holes, pull the assembly apart and drill one sprocket and the spacer to just clear the bolts and tap the other sprocket. Assemble the three pieces with your four bolts making sure your white index marks line up. Having a shaft in the bore while tightening is a good idea btw to ensure concentricity