Guilherme Rambo and Stephen Troughton-Smith have been exploring macOS and have found 'Hey Siri' functionality in the code base with support for multiple user accounts, just as macOS has long supported user switching.
If the accounts are correct, It looks as though the ARM coprocessor takes over the boot process, security, and the FaceTime camera. It also appears that the inclusion of the A10 Fusion allows the iMac Pro to accept the voice command 'Hey Siri' rather than requiring the click in macOS on the Siri icon or keystroke to prompt Siri.
ТThis doesn't mean necessarily that Siri will understand multiple users speaking while logged into one macOS user account. But, it may mean that if a user has multiple accounts on a Mac, they should be able to fast user switch between them and have the iMac Pro recognize 'Hey Siri' for each of them.
It is unclear if this functionality will need a coprocessor like the A10 Fusion to be implemented in other machines.