Thanks again for your help.
Multi-master I2C looks really interesting, I just gave it a try connecting 2 Arduinos together as masters : works great ... but I2C Arduino's Wire library is blocking : in consequence, if a (master) receiver is not available, sender (also master) is blocked ...
Edit : actually it was a wiring problem (SDA & SCL lines weren't pulled up anymore as the second Arduino was disconnected)