> 2 pipelines running simultaneously with a different SCM commit
I can see no problem with this as long as the shared repository remains within one agent's hierarchy as there can be only one job running at a time. In practice I had a similar arrangement in Jenkins where one could specify the path of the agent workspace and I never had a conflict with thousands of pipeline runs.
> A better approach would be to expose the repo as an artifact from a pipeline and just download that in all downstream pipelines.
I will try this suggestion, thank you for the hint.