Strictly speaking, layering is not really a method for processing, but it’s a common approach to take when designing a bass sound. Layering is an additive approach to designing a sound, because you’re building it by adding different elements together. By contrast, a subtractive approach (such as subtractive synthesis) works by starting with a big sound and taking away the parts you don’t need (for example, by filtering). In the real world, you’ll probably find yourself combining thw two approaches.
Recall the discussion about character and body. Sometimes you might find that your main bass sound has a satisfying body (energy below 100Hz), but not much character (above 100Hz). Other times, you might find that you like a bass sound that has a lot of character, but not much body. Using layering, you can build a composite bass sound that has right body and character for the song you’re working on.
The trick to making this work is to stay focussed (in your mind) about what you’re trying to achieve. Otherwise it’s too easy to create an indistinct mess of sound.
For example, you might have a deep filtered synth bass that sits perfectly at the bottom of your mix, but loses power when the rest of the mix gets busy and doesn’t “cut through”. You might try to make the bass brighter by raising the lowpass filter or using saturation, but then find that the sonic signature of the bass changes too much and you lose the characteristics that you like about it. Rather than trying to make the bass sound brighter, think about layering a second element so that the original sound stays at the bottom of the mix but the added layer adds some more character in the lower mids. You wouldn’t need much – the added layer can be effective even if it’s quieter than the original layer.
Alternatively, you might have a bass with a lot of character in the lower mids but find that it’s not adequately covering the bottom of the mix. You might also find that the level of the bass below 100Hz varies quite a lot (especially if the bassline covers a wide range of notes). Boosting the bass might make the level even more uneven, and reducing the note range of the bass would probably compromise your bassline. Rather than trying to add more of what isn’t working down there (or destroying your sound with a multiband compressor), consider adding a new layer to cover the bottom of the mix. That way you can focus the original layer on the range where it’s strongest (the lower mids, or wherever the character is). If the bassline has too wide a range, you might even simplify it for the lower layer, so it is more focussed and sits better under the mix.