SAMD21 Mini Breakout Master /Slave on I2c

Hi! I’m trying to link two Samd21 Mini Breakout in order they could share data on I2c bus. There is a lot of examples on Internet about Master/Slave connection of Arduino boards, but none of them does work connecting two Samd21 boards. Of course I replaced in the code Serial to SeriaUSB , but nothing happened. Sometimes in the SerialUSB monitor I see some data but they are wrong and not understood. Which is the trick?

Thanks very much