One of them would basically have me running "dualies" in the back, so even if one tire pops I still have the other, etc. It would use a double rim, with independent tires and tubes on each, but both laced to the same hub.
This would require a wider hub and axle, so I'd need to modify an existing one to lengthen it. The hub should be easy to do, but I'm not totally certain about the axle. I think I would have to do the axle first, and in addition to welding a section of another axle into the middle of it, I'd also want to lathe that down to flush surface and then weld some tubing over that that is longer than the added axle piece.
Then I would use one of the thicker-bodied hubs, cut it in half, place the axle between them (since it will no longer fit thru the axle hole from outside), and weld them together with proper diameter tubing sleeved over the outside of the central hub.
I'm not sure if everything would remain as hard as it should after doing that, so it might not work as well as I'd like.
I'd need to use longer spokes for some of the cross-lacing, and I have some older likely 700c wheels that might have the right length. I am not certain how it will affect wheel strength to do this kind of lacing, where every other spoke will go to opposite rims.
Another option is if I have any hubs with enough space between holes to drill new spoke holes in them, I could drill a complete new set of holes in each flange, and lace up each rim to it's own set of holes.
A third option is to weld a second set of flanges on the hub, and give each rim it's own pair of flanges, just as if it were two wheels physically joined.
I'm not yet sure if I should weld the rims together or leave them so they can individually flex to handle loads on their own--I suspect the latter would be more effective.
The wider effective tire would probably make straightline riding a bit more stable, but introduce some funkiness in turning. Double contact patch means double rolling resistance, but it also means double drive power and double braking power.
Well, that is, if I also build a rim brake that's twice as wide. :) Or come up with some way to build disc brakes out of my junkpile of regular brakes, 5.25" harddisk platters, and other stuff like that.
I'm still pondering it, and may never even attempt it. More web research is in progress to see if anyone else has tried this and hopefully posted their results.