Control I2C relay board from two Arduino Uno how can I make multi master Arduino device to work with single I2C device? 
As both the master device are 20 m away from Slave device, 

I am using the I2C relay board to connect with 2 Arduino boards and need to connect I2C device and create the two way switching technique from this. 

Any suggestions here will be a great help.
What you want is to operate in multi-master mode.  This is a supported configuration of I2C - however the Arduino API has no support for it.

It involves each master identifying if a collision has occurred and stopping transmission if it has - to then retry transmission later on.

The Arduino hardware can do it, but the software can't. You would have to bypass the software and control the I2C interface yourself (or locate a better library than the Wire library that supports multi-master).