Bookmarking for later, maybe this guy's solution from 2013? https://michael.bouvy.net/blog/en/2013/05/25/arduino-multi-master-to-master-i2c/ Hmm, with one small adjustment I can get it to compile in modern Arduino for the UNO, but not for the ESP. Error reads: Wire.onReceive(receiveI2C); ^ exit status 1 'class TwoWire' has no member named 'onReceive'