Arduino i2c communication between another arduino and a sensor
Can the following scenario exist?
  Yes. Such a scenario is called Multi-Master.
  
  If such a scenario is possible, then how would the pseudo code for microcontroller 2 receive-event be?


MCU 2 would just be set up as any other slave. It waits, as normal, for an instruction in whatever for you like. Once it receives that instruction in then switches into master mode and makes a normal request exactly as you would from MCU 1 if you wanted it to read the sensor.


On receive:


Set "run as master flag"
Disable slave mode



In loop: 


If "run as master flag" is set:


Perform request to get data from sensor
Clear "run as master flag"
Enable slave mode



Running a multi-master system does have its own set of caveats and things you have to watch out for. You can read more about it here:


https://www.hackster.io/chipmc/arduino-i2c-multi-master-approach-why-and-how-93f638