[GuidoK]

Yes but that single vs double turbo is probably due to that the one gives way more torque in general than the other. Obviously 1000Nm at 3krpm is a bigger strain than 500Nm at 6krpm

But if an engine can stand for example 500Nm of torque at 6k rpm, it can certainly stand 500Nm at 3k rpm.
On high revs everything is more at risk. There is way more heat, so the cooling system is more under stress, heat also weakens parts. Ignition timing is earlier, so the blow the piston gets is less precise etc etc.
It's a logical step to go and turbocharge engines to get more power instead of increasing revs and displacement.
That turbocharged engine is usually lighter in build, yet it gives more torque... at lower rpms.
So I don't agree with your initial statement. I see no reason why a certain amount of boost (say 15psi) would be more harmful at 2k or 3k rpm than at 6k rpm. I think that an engine can stand more boost and torque at lower rpm's than at higher rpm's. How much more I don't know, there is probably not a set value for that.