First, to answer your question, yes there are several ways to use more than 1 board. All of those ways involve trade-offs, so which would work depends heavily on the details of what you're actually doing.
With piezos, there are 2 basic approaches to capturing the height of the peak that occurs when the sense a mechanical shock. The way that is simple hardware-wise requires quite a lot more work on the software. You need to sample the waveform at a consistently high speed, so you don't "miss" the moment when the voltage is at or very close to the peak. Teensy's ADC is very fast relative to the speed of those waveforms, but if you try to time-mux is between 20-some signals, you're spreading things pretty thin. You also have some processing of the data to do, especially so you don't mistake aftershocks and small discontinuities in the waveform rise to the peak as false readings. You can find example code in File > Examples > Teensy > USB_MIDI > Piezo_Drum.
The other way involves building much more analog hardware, but then is greatly reduces the need for speed and post-processing in software. This involves a sample and hold circuit which tracks the highest peak. Your code can read it at a leisurely pace, where the only timing that's really important is resetting the circuit quickly after reading, so you don't "miss" a new peak. About 26 years ago I built an 8 channel MIDI drum machine using this technique. Here's some info:
https://www.pjrc.com/tech/midi-drums/sch-ana1.html
Teensy's ADC is so much faster than that ancient chip, which admittedly wasn't all that fast even in 1991. Back then the fast sampling approach really wasn't feasible. Today, it is. Still, if you prefer the idea of analog circuits and simpler software, this would be the way to go.
Regarding using 2 boards, if you go that route, I'd suggest serial communication for this particular project. Assuming you don't send too much data too quickly, which really can't happen if your code is properly waiting for aftershocks before watching again, with serial you can just use Serial1.write() and the data goes quickly into the transmit buffer. Then your code can keep working on monitoring the inputs. Likewise on the other board, it's probably busy reading inputs. After each scan, you can use Serial1.available() to see whether the other side has sent data, which is sitting in the receive buffer. Presumably you'd send MIDI or trigger the audio library or do whatever else you're going to actually do with the data, both from the measurements and whatever you got with Serial1.read() from the other board. The buffering and 1-way data flow will make this project easier, if you build it around multiple boards.