The I2C Master / Slave Controller IP Core interfaces a microprocessor via the APB system Interconnect Fabric to an I2C Bus. The I2C is a two-wire bidirectional interface standard (SCL is Clock, SDA is ...

for transfer of bytes of information between two or more compliant I2C devices, typically with a microprocessor behind the master / slave controller and one or more master / slave devices on the I2C ...

NOTE: it turned out to be very important to connect GND of both esp32 (just connecting them via USB to the same Computer is not enough!) ...