Look at this link, where somebody describes in detail how to change addresses on this device.
After having changed the addresses (one by one!) of your 2 sensors, hook them up to your Arduino on the I2C bus and run one of the I2C scanner sketches (link to the I2C scanner sketches - start with Nick Gammon's, which will do for most applications) to make sure that your sensors are found under the new addresses.
If you succeeded in confirming the new addresses you can go on with your sketch.