No. As the PD has control of the Pre-construction phase of a project then that would indicate there can only be one. Otherwise he couldnt have control. I am assuming that it is an organisation and not an individual appointed as PD in which case I am sure they could allocate a seperate person to deal with each structure (if it is a question of adequately resourcing the project). Has the client given his reasons for wanting to do this ?

I think you are right, I do not think you can have two concurrent Principal Designers on the same construction project. Although not expressly prohibited in the regulation it is against the spirit and intentions of the regulations. You will get people interpreting the regulations to suit their own circumstances but remind your Client the courts may make the final interpretation for them. Case history tells us you go against the spirit and intention of legislation at your own peril. The accepted industry interpretation is there is only one Principal Contractor and Principal Designer, as there was only one CDM-C. Unlike the Principal Designer role, I would refer your client to Reg 4, 8 where there is scope for two clients and Reg 15 where contractors are mentioned in the plural!

Quote from OP My gut feeling is that he should appoint one for the whole site as this will simplify the management of the project.

Perhaps the OP could clarify as to whether this is a single project or not ?