main thing, since you're starting with layering and EQ, is to throw a decent spectral analyser on each track, and get a general sense of which elements sit in which frequency ranges. after a while you'll start to understand how the frequency spectrum works.
if you've got two elements blatantly taking up the same frequencies, you're bound to notice an effect called "masking" - where two (subs for example) actually sound weaker than just one.
Pure sine waves are only the fundamental frequency, without any upper harmonics. If you've got two sine waves at the same frequency, but one is starting at the height of the wave's crest, while the other is starting at the depth of the wave's trough, the result is they cancel eachother out.
The reason being, is these particular two theoretical sine waves are out of phase. If you have two subs with harmonics, like say, two massive patches youve made or something, chances are their occupying similar frequency ranges, and even if their fundamentals aren't out of phase, depending on the sound, chances are some of the harmonics are.
taking this further - for the record, frequency = pitch. so if you've got two synth subs layered, and you want it to make harmonic sense (as in music theory, not acoustics) they're gonna be hitting the same notes - thus the same frequencies.
for simplicity:
1 + -1 = 0
If you wanna have fun. get two copies of the same track/sample - throw 'em in your DAW, and make sure they start at the EXACT same time. now flip the phase of one of them and listen to the output.
bear in mind i'm not right all the time, and this is just me explaining what i know - which is much harder than actually knowing it - so anyone else that knows about this shit, chime in if you can explain it better! The only way to learn is to always be open to further understanding, and outside perspectives
