Aren't these exactly the bounds? i.e. the range {0, sum of distortions}? The worst case being each buffer distorts by adding exactly some x to the waveform, and the best case being buffer one adds x and buffer two adds -x? I would think that this was sharp, but I may be ignoring some subtlety.

I've noticed that in some cases (2-order IIR implementation of eq filter) the total THD from the two stages is much less than the combined distortion figures.

In this example:

Filter 1: 0.0004% THD

Filter 2: 0.0004% THD

Filter 1+2 cascaded: 0.0005% THD

The upper bound would have been 0.0008% by the above logic. These distortion figures may seem trivial, but I am considering the case of, say, 1/3 octave EQ with 30 filters. How the distortion sums is thus the difference between 0.012% distortion (-78dB) and <.001% (-100dB).

I'm guessing there is no rule of thumb other than to assume that your buffers are doing "similar" things internally, and thus to assume the distortion will be nearly double. Is this the thought process behind your statement?