If I specify multiple git repositories to the git plugin by clicking the "Add" button in the "Repositories" area of the git section of the job configuration screen, then the "git fetch" for each added repository will fail with the message that the remote repository being fetched is invalid.

In this example, I assigned the name "origin-mwaite1" to the second repository.

ERROR: Problem fetching from origin-mwaite1 / origin-mwaite1 - could be unavailable. Continuing anyway
hudson.plugins.git.GitException: Command "git fetch -t origin-mwaite1 +refs/heads/:refs/remotes/origin-mwaite1/" returned status code 128:
stdout:
stderr: fatal: 'origin-mwaite1' does not appear to be a git repository
fatal: The remote end hung up unexpectedly

I suspect the plugin has missed the necessary step of defining the remote repository with "git remote add" if the remote is not already defined in the working repository.

This is only using the Git plugin, not any other plugin. This is not related to any "multiple SCM" plugin topics, nor to any interaction between other plugins and the Git plugin.