I've set up two Multi-Project Throttle Categories, filesystem and postgres, both with maximum total concurrent builds of 0 and maximum concurrent builds per node of 1.
This is because only one job that uses these categories can run on a host at once, if two try, they'll stomp all over each other.
So, I've configured two jobs, both matrix jobs, and both configured with "Throttle Concurrent Builds" checked, "throttle this projects as part of one or more catogories" checked and both the categories defined above checked.
However, when a node has more than one executor, I'll frequently see multiple configurations from the matrix being run on the same node at the same time. Sure enough, they stomp all over each other.