The answer is: it depends. If the access is read only, then yes they can both access the same data at the same time. If they both want write access, then there can be locks on specific bytes ranges and as long as they don't overlap, they can access data. If they overlap, then there will be a conflict and depending on the different type of access controls that may be implemented, there are different possible results.
Standard implementations for NFS usually (assuming that both have the same access privileges) indicate back to the second requestor an advisory level lock condition indicating that another requestor has that data locked. Since it is advisory, it an "on your honor" system to not access it for the second requestor. If that is violated, results can be somewhat unpredictable. Implementations for CIFS generally use "hard locks" where the requestor is not permitted access under any circumstances.
You need to look into how each NAS product implements its underlying access control lock mechanism. They vary widely.
Evaluator Group, Inc.
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This was first published in November 2001