Double top tube is kinda a fad. Evidence of which being that one sees even quality frame builder making them with curved tubes. Depending on how it is done, it can reduce the size, and increase the quality of the triangle, but as to whether that is good for something, better than a single small triangle alone, a different tube placement, gussets, oversize tubes, etc... Is another question.
If it was me it would be a totally unique tube set, the seat tube would not change. You mention the top tubes, but as you say, that is just a thing they did in the past, sometimes to carry advertising, and has nothing to do with long tubes, really. The tube beef is going to depend on ride stuff other than the frame length, like rider size and ability. Basically the main triangle can be fairly similar to a regular frame, but the stays have to be done specially. When you build a frame, for the same riding style, the seat to crank angle (the seatube can vary depending) is fixed, and handlebar reach is fixed. These are biomechanical. You can vary the head tube angle, just so long as reach is not adversely affected. The problem is that the rear parts are made to a limited length. Consider:
1) Sakkit and Brandt approach is to simply use an untrimmed chainstay. This will get you well beyond regular touring bike, but the bike will not be freakish.
2) You can use a construction like a fork, you could probably use a fork. The "steering tube" is welded to the BB. Over length seat stays can be simply extended with additional tapered, or straight tubing. Or wishboned.
3) You can use early MTB construction/Bike Friday construction (Bontrager?) where two sizes of bent tubing are slide fit together, this has a ton of construction advantages, and structural advantages. I don't know who this is going to be built by, but S&S don't just sell their couplers to anyone, so in addition to studying BF tech on stays, you can familiarize yourself with alternative coupler approaches.
4) You can consider how modern Ti, and MTB bikes are built in general. They use straight gage tubing for the stays. These days they sometimes throw in fancy bends, though while it looks cool, it probably does not improve the bike much. But you will see lots of cheap ways of getting your stays done. In the early days of MTBs this was the race winning approach used by many factories.
I would probably not add anything to stiffen the rear triangle until I rode it. Shimmy is not really the test since it has many non-frame causes. I would ride the bike, and then add bracing if and as required. When you are making a really out there bike, particularly with panniers, and lots of BOs, I think it pays to rig it in the white, and play with it, and sort stuff out. Then paint.
The pics are more Arvon bikes. The tandem has a lot of couplers and can be assembled as seen, with a child size rear, or the front section can be assembled to the stays to make a single. I think the blue bike was made for a very tall guy, which explains some of the geometry.