http://www.karan.org/blog/index.php/2009/08/25/multiple-ssh-private-keys
read it
Also, if you’re having problems with multiple ssh keys there a solution here:
http://stackoverflow.com/questions/8786564/cannot-push-to-heroku-because-key-fingerprint
Look for the one that talks about modding your .git/config file
Or if you don’t want to deal with all that:
http://martyhaught.com/articles/2010/12/14/managing-multiple-heroku-accounts/