Why don't microphones have two diaphragms, like loudspeakers have two drive units?
There are many so-called 'dual diaphragm' microphones available. But nearly always these are multipattern mics. One diaphragm points to the front, the other to the rear. The outputs of the two are combined in varying proportions to create all the polar patterns from omnidirectional to figure-of-eight.
Other than that, mics have just one diaphragm to handle every frequency from 20 Hz to 20 kHz. Put that another way - one diaphragm handles sound wavelengths from 17 meters all the way down to a teensy 17 millimeters. Incredible.
In loudspeakers, it was found right back in the very early days that one drive unit could not handle the full range of the audio spectrum. So a large drive unit - the woofer - handles low frequencies, a small drive unit - the tweeter - handles high.
Why shouldn't it be the same with microphones?
Well, things are not really the same. A small microphone diaphragm can pick up frequencies as low as you like. The idea that low frequencies require a large diaphragm mic is a myth. Or maybe it just doesn't look right putting a tiny mic on a double bass. It will sound alright though.
It is true however that large diaphragms struggle with high frequencies. They are fine(ish) when sound approaches from head on. But sound coming from an angle can make one side of the diaphragm move inwards while the other side is moving out. Plainly that is not good, and it results in a poor off-axis frequency response.
But a large diaphragm does have an advantage. Because it is in contact with a large number of air molecules, their random motions are averaged out, resulting in low-noise performance. A large diaphragm mic will, all other factors being equal, be quieter than a small diaphragm microphone.
So it does make sense to use a large diaphragm for lower frequencies, and then at a point where the large diaphragm cannot cope, crossover to a small diaphragm that can perform better.
There is another advantage too - a large diaphragm microphone will become progressively more directional at higher frequencies. A mic with two diaphragms can have more-or-less constant directivity right across the frequency range.
But does anyone know of a mic with two diaphragms, one for low frequencies and one for high?
Yes, there is at least one - the Sanken CU-41. This mic is for the purists, with cardioid pattern only and no attenuator. But I know from personal experience that the sound is amazingly smooth and clean when used in a natural acoustic environment.
So, it's over to the microphone manufacturers. There is no shortage of mics of all kinds on the market, but with two diaphragms, one for low frequency and one for high, there seem to be very few.