suggestions for a ~20cm 2 way wireless communication between arduino like devices
The most economical two way general communication often used for short path &lt;1m  uses one or more optical transceivers such as IRDA and IRDA2 depending on bandwidth and nearby reflections or directional wide angle coverage.

https://www.digikey.ca/products/en/sensors-transducers/irda-transceiver-modules/538?k=IRDA&amp;stock=1