update: Plus, thinking about it, in the situation you describe the A transaction should lock the sequence table until it is complete, and the B transaction would have to wait until A completed and removed the lock.
Sequences are an oracle construct that are NOT tables. You cannot lock a sequence. This is, in fact, the whole difference between a sequence and a table that you just select from, increment, and then update*. If sequences did
behave transactionally, it would lead to lots of deadlock problems and MASSIVE inter-session contention (which is why you'd get so many deadlocks).
* in truth, it would be possible to implement an oracle sequence using a table, but only via another oracle-specific construct called an "autonomous subtransaction". An autonomous subtransaction is a transaction which commits or rolls back independantly of the transaction which invoked it. The point is, you *must* commit the incrementing of the sequence, *even if* you roll back the insert. Otherwise only one database session at a time would be able to have an open transaction inserting into a given table.
Not an editor command: Wq