The ATmega328P supports multimaster operation on the I2C bus but that may not be supported by the Wire library so you may need to manipulate the registers directly. Read the "2-wire Serial Interface" part of the datahseet. It's only about 30 pages. 
The ATmega328P supports multimaster operation on the I2C bus but that may not be supported by the Wire library so you may need to manipulate the registers directly. Read the "2-wire Serial Interface" part of the datahseet. It's only about 30 pages.