There is only one manufacturer taking responsibility for each item of pressure equipment, who chooses one module (or combination of modules).
The conformity assessment is related to an item of pressure equipment and not to the parts considered alone.
It is the responsibility of the pressure equipment manufacturer to obtain from his sub-contractor the information and documentation required for the application of the module chosen. Depending on the module, the notified body could be required to visit the sub-contractor site, and it is the responsibility of the pressure equipment manufacturer to ensure access. If relevant work has been performed by different notified bodies at the sub-contractor site, it should be taken into account.
See also “the Blue Guide on the implementation of EU product rules”, chapter 3.1