A singleton bean cannot use both container managed and bean managed concurrency.
I think any bean (singleton, stateless and stateful) cannot use both container managed and bean managed concurrency.
A stateless or stateful bean are not shared by more than one client at a time, so I think they don't even need container/bean managed concurrency control at all. It won't make sense to use either or both managed concurrency control for a stateless or stateful bean.
Stateless session bean only serve one client at a time.
This means that for the duration of the call this particular instance of stateless EJB will not be used to fulfil other requests.
Once the call to stateless EJB is completed, the bean returns to the pool and becames available for future calls.
If two different clients send two different requests to the same stateless session bean SIMULTANEOUSLY, most likely (an exception explained in next statement) they will be served by two different instances of this EJB taken (by the container) from the pool of available instances.
Note If the size of the pool is smaller that the number of simultaneous requests, there might be no new instance of stateless EJB available to fulfil the request, and so if it happens the container will most likely block the incoming request until an instance becomes available.