MPU-9150: Procedure for Directly Accessing the AK8975

Hi,
I am capable to read data from gyros and accelerometers of MPU9150 with a microcontroller connected on I2C pins 23 and 24.
To read device ID from AK8975C (value 0x48 on reg 0x00), i have:
- set I2C Master Mode off (Reg 0x6A, bit 5 = '0')
- set I2C Bypass (Reg 0x37, bit 1 = '1')
but the microcontroller, when send the address 0x0C, do not receive the acknowledge bit from AK8975C.
The same software works well if I connect pins 23 and 24 (primary I2C) to 7 and 6 (secondary I2C) externally to the MPU9150, tush I must be doing someting wrong in bypassing primary and secondary I2Cs using the internal switches.
Which is the correct procedure?
Any help would be appreciated, thanks in advance.