Even if the user and host are the same, they can still be distinguished in ~/.ssh/config. For example, if your configuration looks like this:
Host gitolite-as-alice
HostName git.company.com
User git
IdentityFile /home/whoever/.ssh/id_rsa.alice
IdentitiesOnly yes
Host gitolite-as-bob
HostName git.company.com
User git
IdentityFile ~/.ssh/id_dsa.bob
IdentitiesOnly yes
Then you just use gitolite-as-alice and gitolite-as-bob instead of the hostname in your URL:
git remote add alice git@gitolite-as-alice:whatever.git
git remote add bob git@gitolite-as-bob:whatever.git
Note
You want to include the option IdentitiesOnly yes to prevent the use of default ids. Otherwise, if you also have id files matching the default names, they will get tried first because unlike other config options (which abide by "first in wins") the IdentityFile option appends to the list of identities to try. See: https://serverfault.com/questions/450796/how-could-i-stop-ssh-offering-a-wrong-key/450807#450807