In CofD, in the "Demon Storyteller's Guide", there is a new embed called "Partioned Memory". It allows the ability to support 2 cognitive functions simultaneously. Does that mean a demon can manifest 2 powers at once?
Depends on the Embeds
At their core, an Embed is just leveraging an unseen rule of reality - consider Knockout Punch. There's a "special way" to hit someone that renders them unconscious.
Now, the Embed in question, Partitioned Memory, states that a Demon can run simultaneous different mental tasks concurrently.
It is beyond the ability of Demons to hit someone without any sort of physical movement - ergo, Partitioned Memory should not allow multi-tasking Knockout Punch.
What would likely be a good litmus test is whether the Embed can be performed without any sort of movement at all.
For example, setting off a building's fire alarm (Fire Drill) while programming a crow with a missive (Animal Messenger)? This sounds feasible.
Urging someone not to smoke (and using Devil's Advocate) while also trying to win an argument (while employing Rhetoric)? This is a bit more of a muddy case; both involve conversation as the medium of the Embed; whether both can be fulfilled in the same action is a much tougher call.
Finding a key (using In My Pocket) while marking someone (using Tag and Release)? These both require overt physical action on the Demon's part - so mental multitasking won't help here.
Bear in mind: "Embeds are always reflexive or instant, never extended" - from the sidebar on DtD p. 124
An instant action takes about 3 seconds, or a single round.
Partitioned Memory isn't a standard Embed - it's designed with Silent games in mind - these emphasize subtlety. The need to pack multiple Embeds into the same round could well conflict with the type of game this Embed is intended for. If you are in rounds, characters are likely performing physical actions; if not, there's little to be gained from knowing someone's fear (Sum of All Fears) at the same moment as one of their aspirations (Heart's Desire) rather than after a 3 second delay.
So my personal opinion would say: When you want to use it, it's probably for Embeds it can't apply to; when it can apply to your "Embed queue", there's little to gain, so I would personally disallow in general, but entertain exceptions.