Currently it is only possible to have a single job using the gitlab-branch-source-plugin for a repository due to the fact, that the context of the status updates is always 'jenkinsci/(branch|mr-head|mr-merge|tag)'.
I see the reason in an ideal world, to have one job per project, but in real-life I stumbled to often over exactly that issue (previously on github projects).

We have currently two use-cases for multiple jobs per repo.

• First: Large project which has troubles with flaky UI-Tests. So UI-Tests are done in a separate Job to see clearly, that at least the build, code analysis, and the deployment works fine, regardless of the flakiness of the tests. So it would be great to have a 'jenkinsci/ui/branch' and a 'jenkinsci/build-and-deploy/branch' context.
• Second: A project which consists of different modules in one repo, where every module has it's own multibranch-pipeline. Here a 'jenkinsci/server/branch' and a 'jenkinsci/client/branch' would be of use.