Even if only one turbo is working I would have thought it would produce more performance than a VW diesel. Stiff/seized turbine/compressor shafts, blocked intercoolers or loose intercooler pipes, waste gates seized open will all neuter performance. If you can hear a whine when building revs indicate that the turbine blades are going supersonic and that one or more turbos are working. If, at the same time, you can hear a rushing sound, that probably means that boost is leaking from a split pipe or loose connection. If you can hear the whine but no rushing sound that could indicate that the waste gates are seized open. If the car performs like a normally aspirated engine of a slightly less capacity then the air intake plenum probably isn't blocked. Trial and error is a sound way to locate the problem, either that or an inspired guess! Good luck, the thrill of restored performance should be your inspiration!