I don't want Build Config A and Build Config B to run at the same time. This is because they share the same resource which cannot be accessed simultaneously. However each build config is run by a separate agent so it is possible for them to run simultaneously.
Instead I would like one build config, when triggered, to wait for the other to finish if it is running. For example if Build Config B begins to run but Build Config A is already running, then B would wait until A finishes and then B would run.
I don't think a snapshot dependency will work because that assumes one config has a dependency on the other which is not true in my case.
For a quick and dirty, just make a rule on the compatible agents tab for the project. That they have to run on the agent name containing xxx ( whatever that is ) then those builds can only run on that agent and will never run at the same time.