Unfortunately, you do not understand, because you are continuing to post incorrect information.
Decibels are defined as 10 log(Power ratio). Since sound power is proportional to the square of the pressure level, you can also write it as 20 log(level ratio).
However, YOU CANNOT JUST ADD WAVE AMPLITUDES AS SCALARS. Waves have amplitude, frequency, and phase. If you look at the superposition of two pressure waves of the same frequency and amplitude, then the pressure level of the superposition depends sensitively on position (look up wave interference for the details). The superposition is NOT just a wave with twice the amplitude. In some positions, the two waves will cancel out and result in zero power, and in other positions, the waves will constructively interfere and result in four times the power.
This is all fundamental physics of waves and is covered by any Physics 101 type course.
Now, as to why two 20dB fans will add to about 23dB (rather than 26dB as you incorrectly stated), it has to do with frequency variation and the definition of decibels.
As I already wrote, two fans will not have exactly the same frequency, even if they are both controlled by a good controller. No one tries to make fan controllers with extremly high frequency accuracy, and even if they did, the random variations in the fans motors and bearings, as well as the chaotic air flow, will usually result in pressure waves that have enough variation in frequency that you don't get much wave interference. If the fans were producing identical frequency pressure waves, you would be able to move your microphone around and find positions where there was no sound at all, and other positions where the sound power is 4 times that of a single fan alone. I don't know about you, but I've never noticed that effect with two fans. Two acoustic speakers, definitely. But two fans, no.
Anyway, the interference (or lack thereof) of the pressure waves produced by the fans is mostly irrelevant to the decibel question. As I already stated, decibels are defined as 10 log(Power ratio). Double the power gives a +3dB increase in decibels. And two fans create double the sound pressure power of one fan. So two 20dB fans will result in 23dB of noise.