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.