well to achieve this, i believe you can start using the the service provided by SAP known as SAP LogOn Load-Balancing( SMLG ). The details are not enough to clarify your matter. Please mention for what purpose or for running what king of report, you are looking for this feature? For example in Transport Requests, for this matter can be achieved by using Synchronous Mode.
Another thing is that the fellow users who are supposed to get locked as per your words, will they get any notification for that or else that matter can cause a hamper if executed such thing in between their jobs.