Glad you lived to tell the tale!
Knowing what I know about carbon rims, I would NEVER use them on a tandem with rim brakes. My neighbor who races experienced a rim blowout similar to your experience on his carbon SINGLE road bike. He's pretty light too. And I think a teammate of his had a similar experience. I think they were no-name carbon rims, which raised suspicions, but still. The material simply is not made to withstand high friction and head loads. Aluminum and other high heat conductive metals are a much better choice. So, knowing this, I cannot imagine why ANYONE would trust basically a plastic rim to withstand high and prolonged braking heat loads on a tandem. It simply does not make sense. Sure, "raw carbon" can take high temps, but no one uses ONLY carbon!
I'd say that good advice to any tandem riders would be to only use carbon rims with disk brakes.
Thanks for sharing and as I said, it's REALLY good you survived this failure with nary a scratch!