[BiturboS4] Turbo change in pairs - can someone explain me why ?

[David]

Ok, I understand that if your car has 100 000 miles, and one turbo dies,
chances are that the second one will follow in a short time. I understand
that and old turbo will have maybe less efficiency than a new one, and
mixing them would - maybe - corrupt the balance of the work of the 6
cylinders. But if you have the one which fails fixed, and the other one
carefully checked, by a specialized workshop, what's the problem ? That's
what I'm planning to do, and it should be much cheaper than changing both of
them.
David. S4 Avant Abt - Barcelona - Spain.