Git with different repos for pull and push ?

Hi,

I'm working on a project where we have one Git repository that is used to pull updates and another repository that is used to push changes to.
In the "Git -> Pull..." dialog I can select the remote repository to pull from - however, in the "Git -> Push..." dialog, I cannot select the remote repository.
There is a dropdown field at the bottom of the dialog where "origin" is selected - but the field is disabled and I'm unable to select a different repository.

Is this a bug or is it just not possible to select a repository to push to ?

Thanks,
Christian