> External consistency states that Spanner executes transactions in a manner that is indistinguishable from a system in which the transactions are executed serially, and furthermore, that the serial order is consistent with the order in which transactions can be observed to commit. Because the timestamps generated for transactions correspond to the serial order, if any client sees a transaction T2 start to commit after another transaction T1 finishes, the system will assign a timestamp to T2 that is higher than T1's timestamp.

Of course there is always the edge case where two commits have the same commit timestamp. Therefore from the perspective of Spanner, they happen simultaneously and there is no way to determine which happens first. But there is no need to. There is no causality relationship between them. If you insist, you can arbitrarily assign a happens-before relationship in your own code and nothing will break.