I suggest for easing the dev process that you use regular Arduino boards, so you have easy serial monitoring for informational purposes and way enough resources so that isn't an issue.
Maybe even boards with multiple real UARTs. But serial may not be the best for this exact inter-board communication need. You could use serial in a manner such as to allow exchanging at a somewhat low level the serial implementation for one that used SPI or I2C, for examp,e.
Later if you still want or need to, take the working sketches and embark on the heroic struggle which will be getting them to work on the less powerful chips or boards.
But I am with any who said or hinted that throwing more microprocessors at this may not be necessary or the best way to proceed. You have to marvel at what ppl can do with only one.
a7