NOTE: My hetero-rep experience is limited to some ASE-to-Oracle table-level replication from a couple years ago, so fwiw ...
If repserver is sending the same exactly formatted string in the INSERT command to both standby databases then this would tend to imply the same translation is being used.
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Can you manually (eg, via sqlplus) run the same exact INSERT statement (see the error message) against both Oracle standby databases?
If one fails and one succeeds then this would tend to indicate (at least to me) that the issue is with the Oracle configuration ... ?