I’m working on a project that uses devices that have overlapping i2c addresses. In more recent raspberry pi instances they changed how this works and there is quite a bit of confusion on forums about how to do this. Here’s your 2015 update for using NOOBS as a starting point:
Add these two lines to /boot/config.txt:
echo dtparam=i2c_arm=on >> /boot/config.txt
echo dtparam=i2c_vc=on >> /boot/config.txt
append bcm2708.vc_i2c_override=1 to /boot/cmdline.txt
WIth this, you can use both i2c, pins 3,5 and 27,28. Keep in mind you may need a pull-up for pins 27,28 and your i2c setup, where 3,5 have them on-board.
root@raspberrypi:/home/pi# i2cdetect -l
i2c-0 i2c 3f205000.i2c I2C adapter
i2c-1 i2c 3f804000.i2c I2C adapter
root@raspberrypi:/home/pi# ls -ld /dev/i2c*
crw-rw—T 1 root i2c 89, 0 May 27 01:08 /dev/i2c-0
crw-rw—T 1 root i2c 89, 1 May 27 01:08 /dev/i2c-1