That both tires had deformities seems unusual and in my mind would suggest a tire issue and not a rim incompatibility. That so many tandems run 18-622 rims with 28mm tires -- many of them being Conti Gatorskins -- also seems to suggest a tire-related problem.
As to whether or not it was a manufacturing defect or something that happened to your tires after they were produced, if others have reported Conti sidewall failures then manufacturing defect becomes more likely.
Absent that, perhaps the tires were mishandled before mounting (i.e., wire bead folded by accident) or is it possible that you ran through or over a road hazard with both front & rear tires that damaged the tire sidewalls? In regard to the latter, there would likely be some dings in the rim adjacent to the point where your tires failed.
Just some thoughts...
However, I would agree that if you expect a goodly amount of your riding to include rugged roads in all weather conditions and at night, a set of wheels that would support 28mm as well as 35mm tires would be a better choice, e.g., Velocity Dyad or similar.
Last edited by TandemGeek; 08-13-11 at 01:06 PM.