To quote Jobst Brandt :The high temperature blow-off seems to be a combination of effects
that include primarily increased pressure from heating and the
softening and slipperiness of the tire bead that heat causes. In any
case, enough tandem riders have overheated tires that exploded off the
rim, that it is a known phenomenon. Because we don't know the
temperature of the air in the tire that exploded, we can't make an
estimate of the pressure. One thing is certain from my experience and
measurements I have made, is that the rim gets hotter than 120 deg C
and that steam can be generated from water inside a hollow rim. I
don't know how hot a tandem rim gets.
I think that this link has some helpful if somewhat anecdotal information.
Thank you Twizzle