Exceptional car, congratulations.
In any case, if the cooling sytem for engine and bi turbo was drained 3 years ago or more, you should definitely have both circuit drain.
A Bentley specialist should do it, draining the intercooler mono turbo is not easy and air is a dnager ofr those engine.
Draining the bi turbo is more specific and Bentley has a special procedure and coolant.
If the engine and bi turbo are not drained and some air remain, you may create serious dammages to the turbo and /or the engine.
Continental R. 97