Is this possible?
The I2C specification does cater for multi master bussed. So in theory it is.
However there are two snags:-
1) I don't know of any multi master software.
2) The Lego uses a 3v3 I2C bus and the sensors might need a 5V bus so you will need some hardware bus level shifters.
Can i do both simultaneously?
You can do nothing simultaneously on a single processor, you do them sequentially very fast so it looks like simultaneously.
I know thath for I2C communication i have to use the pins 4 and 5 of the analog input,
It is not those pins on a mega it is pins 20 & 21