Thank you very much for your long detailed reply, very appreciated. I need to think about it for a while and see if I truly understand everything.

Yes you have understood my drawing correctly, shift 2.5 degrees to "only" end up with two spoke lengths and as small error as possible when compared to a normal 24->24 build.

The reason for using a 36H hub and 24H rim is that the hub is a DT Swiss 540 145mm tandem hub, they only make them in 36H and 40H (it's more of a heavy-duty touring hub rather than racing hub), the rim will be a custom-reinforced 55mm deep section carbon rim intended for aerodynamics. I aim to use Pillar Wing 23 aero spokes to gain enough stiffness for for tandem wheels with that low spoke count. If I judge this project unfeasible plan B is to try to get hold of a 28H tandem hub from white industries or chris king, but this is supposed to be a budget project and here in Sweden the DT Swiss hubs are much cheaper, and another advantage of using the 540 is that the same hubs will be on the gravel wheelset so I can switch the tandem between gravel/touring and tarmac racing mode without having to realign disc brakes etc.

I also find this problem quite interesting from an engineering standpoint, so if I come to the conclusion that it can be done I'll probably do it, even if it's wiser to pay up for a 28H hub and get make a normal 28H build. I'm not doing 36->28 as that becomes even messier, but only 24 spokes on tandem wheels is not something that I would generally recommend, but the reinforced rim plus the heavy-duty aero spokes should do it (Rolf Prima is doing it in their commercial tandem wheels). If I get a new hub, it will be 28 spokes though, which is the smallest I have found 145mm tandem hubs do, and then only Chris King and White Industries premium hubs, and probably made to order. Normally racing tandems use standard hubs so you can easily get any size you want, but our Cannondale is more of a do-it-all racy tourer so it uses the tandem-specific 145mm width. So why not just run with 36 spokes and call it a day? An obvious question so I thought I better answer that too: no commercial aero wheel design use 36 spokes, and I managed to find some data: While the loss of going from 24 to 28 (aero) spokes is not that bad (most would call it negligible), 36 is past a point when there is quite significant detrimental effects to aerodynamics, and why go through the effort to make fast aero wheels with when wasting a big part of the advantage by using too many spokes.

Anyway, it will surely be a tough build, not only because of the different spoke lengths, but I also plan to exclude the center channel holes to insert nipples for even more rim robustness, so there will be magnet/wire. I can live with that though. This is a winter project so it doesn't hurt if it takes some time :-)

What I am worried about that it would be almost impossible to get this wheel trued and tensioned properly due to the 2.5 degree shift thing... it seems like the information for figuring that out is in your answer, I need to think more about it though, I'm not wheel building expert so I don't have that deep knowledge about exactly what happens when tensioning a wheel.