Accessing two MPU-9250 DMP
Sorry, but I'm not 100% sure of what you are trying to do, I don't understand 'both imu the' in your question.  I think you are trying to use two identical boards at the same time.  (If not tell me so I can edit my answer)

You will need to modify the library so the begin function takes a parameter.
That parameter for MPU9250_DMP::begin then identifies the address of the device, this will need passing to Wire.begin() I think (you'll need to check the Wire.begin() function, I can't).

That should let you create two different instances of the class.

MPU9250_DMP apple;
MPU9250_DMP banana;
apple.begin(0x68);
banana.begin(0x69);