I'd like to see the details of the wind tunnel test which shows riding two up makes both riders slower. Is that in comparison to riding by themselves? If so, that's a surprising result to me. On the other hand, if the claim is that riding two up is slower than one behind the other, that would be easier to believe.
Two cars running side by side are slightly slower than a single car, assuming the same output power. The dynamics of air flow creates a lower pressure between the cars which works increase the drag slightly. Probably the same principle with two bikes, just less of an effect.
Yes, that was essentially the gist of it. Note this was observed by the good folks at Alphamantis who do their aero testing on a track.
From p.64 of the April issue of Road Bike Action:
"For the drafting test, Dr. Edwards remained in his regular road position, hands on hoods, and was joined on the track by Nate Koch, a top U.S. pro track racer. Riding at a constant velocity just behind Koch, Dr. Edwards recorded an average CdA of 0.200. This was a CdA of 0.095 lower than riding alone, resulting in a savings of 95 watts when riding at 30 mph. Next, Dr. Edwards and Koch rode side by side, which yielded some surprising results. Dr. Edwards recorded an average CdA of 0.324, or 0.029 higher than riding alone.
“Side-by-side riding doesn’t have the cyclists share any of the low-pressure region behind each rider,” explains Froncioni. “Each individual has his own low-pressure zone pulling him backwards. In addition, the flow of air is forced around a wider frontal area, which leads to an even bigger low-pressure zone behind them.”