How to access multiple repositories in CI build?

If you are running gitlab version 8.12 or later, the permissions model was reworked. Along with this new permission model comes the the CI environment variable CI_JOB_TOKEN. The premium version of GitLab uses this environment variable for triggers, but you can use it to clone repos.

dummy_stage:
  script:
    - git clone https://gitlab-ci-token:${CI_JOB_TOKEN}@gitlab.instance/group/project.git