I have been happy with the FSA BB-7000/TD MegaExo tandem bottom brackets, they’re as good as any other. The cartridge bearing BBs are not life time items on a tandem. I think of them as I would chains and tires, I replace them when they wear out. 8000 miles is pretty good, likely less cost per mile than your chain or tires. Look for the larger outside cups as opposed to the compact bearings such as you would find in Dura ace. Leisesturm’s advice is spot on.