Two Arduino boards with I2C sensors
There's two basic ways of doing it.  


Send a byte from the master to the slave first to select which sensor you want, then request the data for the currently selected sensor, or
Request lots of bytes from the slave and fill them in one at a time as the requests arrive.


The first way is the slow, but "I2C" way of doing it - basically each sensor is its own register in the slave, and you select which register you want to read from.

The second way is faster, but you have to keep track internally of where you are.

A good compromise is to implement "auto increment" which does both scenarios: if you send a byte it sets the current sensor to read from. Then each read returns the current sensor and increments the sensor number internally in the slave, so the next read will get the next sensor.

That way you have a way to "reset" the sensor index that you are reading from, and it's reasonably quick to get all the sensors, since you just need to do multiple reads after a single write.