In reply toi think what people are warning you about is the phase of the two different types of Low Hz tones.
what you don't want is for them to cancel eachother out. there
are cases where it will not happen and 2 basslines or kicks will work
great together.
i've been in electronics for a couple years, this almost sounds like it
has to do with harmonics. (4 being a harmonic of 2, 8 being a harmonic
of 4. etc. harmonics have a way of canceling out frequencies or tearing
up frequencies that are not harmonics. but apparently 'harmonizing'
with frequencies that are harmonics. so if you take a low frequency,
say 2000Hz, it may sound good with 4000hz, or 6000hz, but not 3650 hz.
- just a theory right now, since i don't know that much about sonic
theory.