This is kind of funny.

With tandems some 'racing' tandems get rid of the additional tubes (lateral tubes) to save weight. Removing lateral tubes on a tandem saves about 400g on the bike. The average racing tandem weighs between 12000 and 18000 grams. The really funny thing is that the couples and tandem teams that own these 'open' design tandem frames swear (from their seat o' the pants meter) that the bikes are just as stiff (if not stiffer) than equivalent tandems with lateral tubes. Apparently when you spend between $6k and $12k on a tandem, its not easy to acknowledge that you bought something that doesn't make sense on paper. However, there isn't really a competitive advantage in having a stiffer more efficient tandem in racing, because the tandem racing cult all seem to buy 'open' design bikes.

Bill McCready of Santana Bicycles has spent serious time and money researching frame designs, and testing comparable designs (overbuilt frames sans lateral tube, standard tandem design, etc.) and determined using benchmark testing and real metrics that the loss of the tubes results in removing "a tandem's lateral tubes to make it lighter, because the resulting bike will have a lower level of pedaling efficiency, it will also be slower. While lighter or weaker teams might not be able to detect the loss of performance---repeated scientific testing shows that no competitive tandem team is so light or weak that they should prefer a frame without laterals." (Source)

The irony being that no one has done more tandem frame testing than Santana, and they still produce frames that are significantly less efficient and noticeably flexier than other tandem makers.

Triangle size is very important on tandems, where the frames are subject to twice the drivetrain forces of a single bike. You can actually see a steel tandem torque itself laterally from the wattage the team produces.

However, there are unusual bikes that have open designs, that are effective. Bikes like Co-Motion's Periscope, and Bike Friday's Twosday. McCready acknowledges why these 'open' designs work: "Other tandems lacking laterals...are frames of exceedingly small stature. In this group I include Bike Friday's "Two'sday" and Co- Motion's Periscope. What these smaller "open" frames have in common is that there isn't much room between their top and bottom tubes." (Source)

So what is all this talk about tandems in a thread about compact geometry? Its relevant empirical proof that compact geometry makes sense, and allows builders to make lighter, stiffer, and stronger bikes with compact geometry than they could with otherwise classic geometry.

Tandems are the ultimate torture test of bicycle frame design. Every production tandem made utilizes 'compact' geometry, or some variant geometry to a great extent. Tandem frames utilize significantly longer seatposts, and typically higher stems than do normal single bikes.

There is a reason for this. Smaller triangles.