As far as I can tell compiz will only allow one keybind as you noticed.
However with a tiny bit of work you can get around this limitation!
First install pycompiz found here. If you have svn you can do this using
svn checkout http://pycompiz.googlecode.com/svn/trunk/ pycompiz-read-only
Install after navigating into the projects folder,
sudo python setup.py install
Next create these two tiny python scripts, and place them in your ~/bin folder.
Then either make them executable (chmod u+x FILENAME) or when setting the keybindings use python SCRIPTNAME.
file: putleft
#/usr/bin/python
import compiz
compiz.call('grid', 'put_left')
file: putright
#/usr/bin/python
import compiz
compiz.call('grid', 'put_right')
Lastly you can create two custom command shortcuts for each of these scripts in the custom commands section of the compiz config (ccsm), and hey presto - two keybinds to achieve the same effect. As mentioned above, if you made the scripts executable, the commands will be putleft and putright, else they will be python ~/bin/putleft and python ~/bin/putright.
Enjoy!