The defaults again:
- defaults:
  name: global
  properties:
    - throttle:
        max-total: 2
        max-per-node: 1
        maxConcurrentPerNode: 1
        maxConcurrentTotal: 2
        throttleOption: project

Using my global throttle settings:
  <properties>
    <hudson.plugins.throttleconcurrents.ThrottleJobProperty>
      <maxConcurrentPerNode>1</maxConcurrentPerNode>
      <maxConcurrentTotal>2</maxConcurrentTotal>
      <throttleEnabled>true</throttleEnabled>
      <throttleOption/>
      <configVersion>1</configVersion>
    </hudson.plugins.throttleconcurrents.ThrottleJobProperty>