A couple of quibbles, first, forks do not have trail. Forks have rake. The combination of head tube angle and fork rake produces trail. The best way to change trail is to change both the head tube angle and the fork rake together. More fork rake with the same head tube angle will result in 'wheel flop', not a good thing. And no, I don't think your analysis is quite correct. Less trail means less stability period. Less at low speed, less at high speed. More trail means more stability at low speed and even more at high speed. Sounds like a good thing until you want to dodge a moose at speed. I should hope your tandem feels more stable than a half bike at speed. The reason it feels so unstable at low speed is precisely the reason it feels planted at high speed. Mass. At very low speed the mass is working against you. At high speed it is working with you. I may not be quite right about this but I suspect that tandems actually need less trail than a given half bike, because that long wheelbase and all that mass could make for a vehicle that was too stable for its own good at reasonable road speeds. You just have to accept the bike designers compromise between reasonable agility at speed vs some amount of instability at very low speeds. Take it as a challenge. I can practically track stand either of our tandems and one of them is has about as much low speed stability as a helicopter.

H