LSM303CTR 3-wire SPI and I2C - conceptual problem
So you will have a circuit like this, where M1 is your 4-wire device and U2 is 3-wire device:



When accessing 3-wire device, you need to send and receive data on the same pin (MOSI). How to achieve this, depends on your MCU. When 3-wire device is sending data, other devices on the bus are not selected, so there's no issue.

According to the datasheet of LSM303CTR, I2C block can be disabled by software.


  In order to disable the I2C block for the accelerometer, CTRL_REG4_A
  (I2C_DISABLE) must be written to ‘1’, while for magnetometer
  CTRL_REG3_M (I2C_DISABLE) must be written to ‘1’.