Changing I2C address
This may not answer your question directly, but another way around the problem, that does not require changing the device addresses, is to use a CD4066 to connect one of the two sensors at a time, like in the Wayne and Layne's Arduino Video Game Shield. Scroll down to the sction entitled Nunchucks. There's also links to schematics and firmware. 

Wii nunchucks (the wireless controller of the famous videogame) show the same problem: they connect to the main wireless controller using I2C, but they all have the same address of 0 (zero).

What the authors of that device cleverly did was to use a bidirectional switch IC (the CD4066) to communicate with one controller at a time, disconnecting the other from the data lines while doing so. Below is a diagram showing the general idea behind their design.



I've built such a device using their open source schematics and it works perfectly with both controllers hooked up at once. Maybe that's an option for you to consider if the setting of the addresses (as suggested by Dzarda) fails.