It is perfectly possible for two resources to take 2 dfferent work queue items at the same time - work queues allow concurrency. If you want to prevent that you should use environment locks in some way. It is impossible for two resources to both lock the same work queue item - if you see that I would suggest reporting it to the support team.
Ensure you are using our Process Template as the basis for all your processes (it should be mandatory in your organisation to do this), and look at the Lifecycle Orientation training solution as an example of a best practice solution. If you do that you will end up with a best of class solution that has best practice work queue handling and exception handling.
