The problem I am trying to resolve isn't so much that Job B must wait for Job A, and is dependant on the results of Job A. That would be a simple precedence.
We have two jobs that should not be run at the same time. Running Job B while Job A is running can lead to strange behaviour and incorrect results, so we want them to be aware of each other. Neither is particularly time critical, so we can afford to delay one and allow the other to finish.