Many Bike Friday's were sold with 3 x 8/9 SRAM DualDrive hubs as OEM. Bike Friday Family Tandems are self coupled, and Tandem Two'sday's both fold and separate (self coupled) without additional S&S fittings. Even in Australia there must be scads of coupled tandems on the secondary market that were either custom built or coupled after purchase. Either way, they are for sale for 1/2 or less of their original prices. Front derailleurs aren't that bad. There is no free lunch. The 3sp internal gear hubs have their own issues. The Dual Drive and their Sturmey-Archer successors exist mainly to provide 20" tires the kind of gear ratios that are standard on 26" and 700C tires OR be used in combination with a standard front double (or triple) to create tractor trailer type gear ranges.