I think it will be an issue of Tx demarcation. Depend on what connection, isolation level and DB (driver) you use the two connections are in the same or different Tx.
I'm not sure whether a no-tx-datasource will non-block read from a DB where a Tx is running with 'isolation serializable'.
So if you try to achieve a higher performance you should think about 'optimistic locking' that will be a common approach for high parallel access.