NEC ECC: Delegation of project manager's duties

At the beginning of the contract the Employer’s Project Manager named in the contract data delegated his duties to another person until the completion date of the project.

Is this a change in Project Managers?

Can both PMs give instructions? (If the original PM gives an instruction is this then a valid instruction?)

Depends how the communication is specifically worded, but delegation would mean as well as not instead of, so this would mean that either person named could carry out the responsibilities specifically outlined within the delegation communication. If the communication said all powers then yes either person can instruct/accept accordingly.