Why S and S Couplings can't be retrofit to aluminum frames.
Aluminum S and S Couplings are currently available only on new Santana tandems. Santana has the exclusive on aluminum couplings and they don't offer a retrofitting service for existing Santanas or other brands. In the future, other framebuilders may offer aluminum couplings on new bikes but I doubt that it will ever be economical to retrofit couplings to an aluminum frame. Even when considering the retrofit of a steel frame, which is easy to do, it is frequently more economical to sell your existing bike and buy a new coupled bike rather than doing a retrofit.
Here are some of the problems encountered when retrofitting couplings to an aluminum frame.
If an existing aluminum frame is modified by welding (couplings welded in), the entire frame must be heated in a furnace for several hours or the newly welded area could fail. This process is very expensive if done on a low volume basis which retrofitting would require. Heat treating isn't required when retrofitting a steel frame.
The builders that do mass produced frames, don't want to interrupt their production to do retrofits yet the small framebuilders, don't do aluminum frames. Most of the authorized S and S framebuilders use only steel.
We don't have the different size couplings required to do retrofitting. We only stock aluminum couplings in the sizes required for use on new production bikes because it would be too costly to produce them in small batches. Since we are only building couplings for Santana, we don't have couplings for the multitude of different tube diameters that would be required for retrofitting other brands. Cannodale tandems, for instance, have a huge boom tube while Santana uses two small tubes.
Since aluminum couplings fit into the inside of an aluminum frame tube being coupled instead of the outside like couplings for steel, the tube wall thickness and inside diameter is also a factor (tube usually has a standard nominal outside diameter while the wall thickness and inside diameter is variable). We would not only need to stock all the different diameter couplings but we would need to have them machined to fit all the common tube inside diameters (we also do this for titanium couplings). This would require us to stock a huge variety of couplings, for retrofitting use only, making them very expensive since we would need to produce them in small batches. Tube wall thickness isn't a problem for steel bikes because the couplings slip over the outside of the tube. For example on a steel frame, a 1 3/8" coupling will fit any 1 3/8" tube regardless of its wall thickness. Since we already make large batches of 1 3/8" steel couplings for production bikes, the same couplings can be used for retrofitting.
I am frequently asked why we don't offer couplings that could be bonded (glued) to the outside of aluminum tubes. Bonding would eliminate the heat treating process and would eliminate the need to stock couplings for all the different wall thicknesses. I like that idea but it would require a significant commitment on the part of a builder to test and qualify a bonding process capable of delivering long term reliable service and I haven't found any builders interested yet.
I suspect that at some point we may have a builder with enough interest to start retrofitting a common bike such as a Cannondale tandem since that is the model most people ask about but I don't see it happening any time soon.