What happens when two users access an Entity Bean concurrently?
So, to answer your question, two users will never access an Entity Bean concurrently.
If you wish to know more about this issue, I would suggest downloading and reading the white paper from Sun: http://java.sun.com/products/ejb/docs.html . Some entity beans may require loopback calls, where bean A is invoked, in turn invoking bean B, which then invokes a method call on bean A. This kind of concurrency is tricky and is best avoided.