How manage multiples modules with SPI?

Using Arduino Networking, Protocols, and Devices
2

You do not need to disable the SD SPI. It is handled by the SD library transfer routines. Your sketch does the same here:

unsigned int readRegister(byte thisRegister) {
    
    byte result = 0;
    
    // select only the MPL115A1
    digitalWrite(MPL115A1_SELECT_PIN, LOW);

// you shouldn't do this
//    digitalWrite(SD_CS_PIN, HIGH);
    
    // send the request
    SPI.transfer(thisRegister | MPL115A1_READ_MASK);
    result = SPI.transfer(0x00);
    
    // deselect the MPL115A1
    digitalWrite(MPL115A1_SELECT_PIN, HIGH);

// you shouldn't do this
//    digitalWrite(SD_CS_PIN, LOW);
    
    return result;  
}

writeRegister() is the same. All you need to do is insure both are disabled at the end of your setup function. Leave those pins alone once in loop().

edit: I modified the code to fit traditional slave select pin settings. Each library takes care of its own.