But those weren't both loaded into memory at the same time.
Thinking about this I've convinced myself that what will happen is the ARM core will periodically check for various statuses (i.e. connect to your imap/pop server and see if there is new mail) and turn on an indicator or perhaps wake the intel cores. Which is to say that the powernap/low power functionality will be restricted to either things apple delivers ootb or to a special API. Not as something that automatically migrates whatever workload is running on the powerful intel cores into a slow motion version of the same thing running on the ARM cores.