As moses said, its all subjective, and separation is a broad term.
Example:
Certain elements that mask each other, are given some separation in the frequency domain, as well as with panning.
Certain elements that are the same, using 2 kicks layered, are given some separation by modifying the attack and release.
on and on...
But i believe you are probably referring to the first example.
Then the issue of sounds getting "mushed" together, causing a cloud of sound..(subjective)
Most DAW's allow "Stereo Dual Panning" which gives you two "Balance panners in the space of one, so that you can control the panning for each side of a stereo signal separately".
There are plugins you can use as tools for this.
Some reverbs have it built in.