When different workspaces are on the same node, and the same Configuration with Concurrent set to "yes" triggers a build, it still displays "WAITING_PROCESS" with the message "Workspace already occupied by another build of the same configuration."
I set the Workspace Directory Setting in the Advanced Settings of the Configuration to "A," triggered a build, then modified the same configuration to "B" and triggered the build again.
QB internally uses configuration id to make sure that only one build of same configuration can run on a single agent (or server). Also configuration workspace directory is not assumed to change during a build. This will cause many issues. For your case, please use multiple agents.