A common metaphor in mindfulness discourse is that observing thoughts and emotions is like being on a hillside watching the clouds go by: if you observe long enough, you'll notice that they just fade away, like puffy little clouds do. Question: doesn’t the act of observation change the nature of the experience being observed? Thoughts and emotions attend to their own objects and attention is a limited resource. Thoughts and emotions cannot “carry on as usual” if the limited resource upon which they depend has been diverted to the cognitive process of observing them. Thoughts and emotions are weakened through observation so they eventually end up as insubstantial wisps of nothing. Nothing here, keep moving.
Awareness tracks attention, nothing more. Attentional capacity is the boundary condition of awareness. If you cannot attend to two things at the same time, you can’t be aware of more than one stream of attention at the same time. You can’t be aware of everything. You have to pick and choose and switch and go back and forth.
It is possible to become so adept at attention switching that it feels simultaneous, especially when the switching is happening on the order of milliseconds. To the extent that attention switching becomes automatized (through overlearning, as probably happens in some types of meditation practice), it may not require all that much mental effort. One just gets in the habit of switching from being deep in the forest to looking at the trees from the outside. Of course, the view within the forest is not the same as the view from beyond its boundary.