Accurate frequency output through external speaker
Every speaker works at every frequency, just that the efficiency falls off outside the rated range.  You can make the cone of a subwoofer move in and out over its maximum travel distance at 2 Hz easily enough.  The problem is that the displacement required for the same sound power goes up with decreasing frequency.

You can make more use of the sound power you do get out of the speaker by tighter mechanical coupling to the microphone.  Mount the speaker on one end of a box pointing inward, and the microphone in the middle of the opposite end, which stays open.  The wavelength for 2 Hz is about 165 m, so a box a few feet accross won't cause interference fringes off the walls, like it would at over a kHz or so.

Even if the speaker is out in the open, moving in and out 1/2 inch, and the microphone is a couple feet away, you have to ask yourself what you expect this microphone to pick up for real that would be louder than that.  If it can't pick up this infrasound with 1/2 inch displacement, what is it going to pick up in the real world?

Another issue is that you will probably need to design your own power amp to driver the speaker.  Normal audio amps have high pass filters that should attenuate 2 Hz severely.