push using multiple account / multiple identity on github / bitbucket

push using multiple account / multiple identity on github / bitbucket

I have multiple accounts on github / bitbucket and one unique
private-public key pair for each account. The problem occurs when I need
to push to the repos created by the different accounts, I will almost
certainly get access denied unless I am pushing into the default account
(the account that I first created).
Is there a way to switch to different credentials before pushing? I am
using Source Tree on Macintosh machine. I'd prefer not to rename
~/.ssh/id_rsa manually whenever I need to push.
Thank you very much for your input!