A good rule of thumb is that if noise source #2 is about 10 dB less than noise
source #1 (9.89dB and some change, if you want to be more precise) it will
raise the combined noise by 1dB, all other factors being equal. Since that's
considered the "just noticeable difference" in this context, in practice any
time the second noise source is more than 10dB down from the first, as a rule
of thumb you don't have to worry about it.

Peace,
Paul