FIR rims as well as Specialized Armadillo & Turbo branded tires have had some independent issues...

The first showed up around 2000 when folks running the FIR rims with both Specialized Armadillo and Turbo / red sidewall tires started experiencing blow-offs and other tire durabilty / failure problems. The 'fix' was a switch to Continental tires. The archives at Tandem@Hobbes are rife with many of the accounts. I'm not sure if they ever fully determined if there was a rim compability issue or not; perhaps someone will know who may have lived through the problem.

The second issue that I recall was a bad run of the FIR rims back in 2001 when a die fell out of spec. The sidewalls of the affected rims would deform & bulge out under pressure where the initial indicator of the problem was unexplained tire blow-offs just as you described. It would be prudent to inspect your rims to make sure there weren't any signs of sidewall fatigue where the sidewall joins the rim bed as that's where the die problem was, i.g., insufficient material thickness.

In any event, the folks at Santana should be able to confirm if the rims that were shipped on your tandem were part of the problematic batch based on a check of your frame's serial number and/or the serial numbers on the rims.