How to set address for using more than MPU9250
This is the type of information which a Data Sheet is intended to communicate.  In I2C mode the AD0 pin selects between two possible addresses,

1101000 for AD0=0 and 
1101001 for AD0=1.  

In SPI mode you can presumably have as many as you like by giving each its own chip select.

That said, using multiple sensors may not be very useful unless your apparatus is flexible - only the 3 accelerometer axis would be expect to yield different values based on position relative to the center of rotations.