How do i send data from a Master to another Master? I am using an ESP32 to receive data from web. 
However in an I2C communication, the EPS32 does not work as a Slave and i already have an Atmega as the Master. 

i have seen sites saying Master-Master I2C Connection is possible however, how do i do this? or are there other solutions to have the ESP32 communicate to the ATMega?
Both your master devices must support Multimaster mode, i.e. Arbitration and Synchronization. See I2C Spec here for a description of the arbitration process.
It's up to you to handle a lost arbitration in your software.


  or are there other solutions to have the ESP32 communicate to the
  ATMega?


Of course. As you considered I2C to be sufficient, UART and SPI are feasible alternatives supported by almost all microcontrollers.