This will help if you don't want to run a monorepo, but many in the industry consider a monorepo suitable for their organisation. The biggest blocker with Gitlab in my opinion is not being able to only run a job if some specific folder was modified (https://gitlab.com/gitlab-org/gitlab-ce/issues/19232).

In places I've worked in before that needed to join concurrent changes across repositories, we would have the build system always build from a super-repo that had the other repositories tracked as submodules. Co-dependent changes were pulled in via a single commit in the super-repo.

This was kind of cumbersome to maintain TBH, and the fact that changes to different repos can be dependent on one another seems to strongly suggest that the code should be together in the same repo. Personally, I opt for mono-repos until I'm forced to change for whatever reason.

Is this not something that is already being worked on? Making commits across even submodules seems to have little support anywhere.