Not all brain activity is unified, I just claim it must unify on the output side - behavior and language.
But this is not a brain or organism only problem. Even a cell must unify in crucial moments such as division and chemotaxis. A cell either divides or not, no mid point. To persist it must unify at some moments in its activity, yet it is a distributed system of many parts.
The core principle here is - what is the unit of selection? the cell survives or not as a whole, the organism also survives or not as a whole, not organ by organ. Selection chooses the mechanisms that are viable.
Idk, I'm pretty good at continuing a conversation while handling the busywork of navigating a cafe. Or even riding my bicycle... The brain is a massively parallel bag of meat. We get some real advantages from focus, but the brain does just fine juggling its many inputs and outputs.