Hi Ben!
I am so sorry these innertube problems have contributed to shortening your summer tour -- how very disappointing!
From what you've written, I gather this is the first time you've had the tube/tire problem on this bike's Rigida Andras in thousands of kilometers, correct?
If the rims have already proven reliable, then I would suspect these tires are slightly large in diameter and this is why it is rolling off the rim. If the rims are new,then I would suspect them of being undersized, as the majority of Schwalbe tires tend to fit tight at the beads, if anything. Tires can roll off rims for other reasons, too, including using too wide a tire on too narrow a rim at too low a pressure. The 1.75" tires on your Andra rims are not a mismatch, so I think you're okay there.
If the tires and rims are both sized to spec, then your problems are due to another source.
As for the tubes...here are some suggestions:
• If you haven't already, try new, fresh tubes of a different brand. Old rubber tubes seem more prone to tearing in my experience, especially if they feel a bit soft and soggy to the touch.
• Check to make sure the rim strip is matched to the rim in *width*. Oddly, I've found many rim strips seem to just miss it for width with Rigida Andras. If they're a bit narrow, the strip can squirm on the rim and cause the tire bead to seat unevenly, contributing to tire and tube problems. I have occasionally gone to a rim strip that fits from the inside of one rim wall to the other, spanning the entire gap. This also has the effect of slightly increasing the diameter of the rim's bead-seat, a help if you have a loose-fitting tire/rim combo.
• Check for rim burrs at the edge of the rim strip if the strip is less than full-width. A longitudinal burr can open up a pressurized tube in much the same way one would use a knife to gut a fish. A slightly too-wide rim strip can do this too, as the reinforced edge can curl up just enough to slice the tube. I've had it happen myself some years ago with a very stiff polypropylene rim strip.
I suspect a pressure problem is contributing to both the tube and tire-retention issues. The rear wheel is the most heavily laden, and this is true in spades for a loaded touring tandem. A goodly amount of torque is required to get the loaded bike going as well, and it is possible to spin the rim inside the tire under starting load -- I've done it when my Dutch touring buddy came stateside and we put some effort into it starting uphill when tire pressures were a bit low. When this happens, the tube -- which is anchored at the valve stem -- pulls a bit as well, and it is possible to open a rent on the rim side of the tube. Worth a check. I think you're on track increasing pressures toward 4bar in a 1.75 tire. That can be pushing it a bit for rim safety in a 2.0 tire, but with two people's weight and a heavy load on a tandem? A bit marginal for a 1.75, methinks. Underinflation can also be a culprit in the tire rolling off, so again I think you're heading the right direction increasing pressure by 1bar.
Now it it at 4bar...are you still having a problem?
If it were me, I'd try the 4bar solution first, then if that doesn't work, I'd swap the tires front to back only for the purposes of checking fit. If the front tire fits snugly on the rear rim and the rear tire is loose on the front rim, then you'll know if the rear tire is oversized or the rear rim is undersized. Of course, if the rear tire fits loosely on the front, don't ride it. A front tire roll-off can be deadly as the tire not only rolls off the rim, it can easily pack up against the brake pad, locking the front wheel as you lose steering control. The result is never pretty.
Others will be along soon with their own equally good ideas, Ben, one of the huge benefits of crowd-sourcing the solution to a problem here on this Forum where the members comprise the greatest resource.
I do so hope you can find the answer soon so you can at least get back on the bike. Perhaps several shorter tours might be possible even if the longer tour isn't in the cards.
Best,
Dan.
