@jake My understanding of the rpi "supercomputer" is that multiple boards communicate through Ethernet and so processing can be shared by programs that support the function.
I suppose that you will need a switch/router that can be powered by the same battery.
Pd is heavy on audio which is why the rpi can run out of steam.
Pd is also a bit heavy through the wish app on window drawing...... especially for arrays...... which is why it is a good idea to avoid redrawing too many guis and keep arrays hidden in tables.
Pd supports two audio processes that can be run on two processors of the same computer through [pd~] .... so the work load can be distributed across two threads.
Audio can be sent through Ethernet obviously (there are some Pd externals that will do that) and I can imagine that you could run multiple copies of Pd on multiple rpi's and pass audio between them through ethernet......... collating I/O in one that has a decent soundcard.
You might be able to keep latency very low..... or maybe latency will not be of concern for your project?
There might be a program that will lock the audio clock between an rpi designated "master" and the others. In windows Dante would do that across a network.
[netsend] and [netreceive] will pass control messages between the rpi's.
For how a single multiprocessor computer deals with scheduling, and the options for fine tuning, this is a good read for all OS's......... https://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/6_CPU_Scheduling.html