Going back a couple posts, so I am caught up.
Ranger dually with Toyota Rims
Cutting out rim centers would be relatively easy, welding in replacement centers, also relatively ease...the problems is finding the centers. And some back of envelope calculation said I need a 350ton press, and appropriate form/die to form my own centers. A little pricey for making 13 or so rims (Summer set, winter set and spare).
Version 1.0 of the tandem doesn't have load balancing between axles - I wanted to review axle placement, etc before I got carried away with details like load balancing, powering rear axle, etc.
I was looking for something like this for load equalization
As OP has indicated trailer springs aren't good choice, they have very small deflection because they are so short - load carrying, not comfort is their forte.
Walking beams aren't good either - on full size trucks, the axles are further apart than spring is long, so you can have full travel for both axles. Note: Full size truck springs are shorter than Rangers, so spring travel is less, again all about load carrying, not comfort. Even Wrangler springs are longer than you would want to space axles apart (and they are about 1' shorter than Ranger springs. And if you don't space the axles further apart than the spring ends, the springs will act as suspension stops. As you can see in 3rd picture there is significant overlap of springs.
The ideal is probably airbags on a 3 or 4 link suspension, but cost got better of me. (I'm not as...frugal as OP apparently, but I drive a Ranger.) With 20/20 hindsight, narrowed Aerostar axles (to Ranger width) would only require sourcing airbags. By interconnecting the airbags, they load share; by adjusting pressure, they provide nice ride empty or loaded...
Bed for my tandem consists of 2 Ranger boxes: Remove bed sides, shorten 1st box between box front and forward rear axle by 6" (corresponds to Wrangler spring moving axle 6" forward), then cut remainder of box off at 48" mark. 2nd box, is mirror shorten box between tailgate and rear rear axle by 6", then cut up off remainder of box at 48" mark. Join 2 box sections together at 48" mark. I'm looking at fiberglass for bed sides - my welding/bodywork skills aren't up to re-joining the bed sides together without LOTs of bondo..