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Adding to ‘s information surrounding the underlying issue of .Q.en for concurrent writes, you might find This White Paper useful. It covers specifically how dataloaders handle mass writedowns which explicitly deals with the integrity of the sym file using multiple processes. Leveraging this will mean you can still benefit from the performance of multiple processes while avoiding any conflicts with the sym file.
This may be an overengineered solution for your use case and a more straightforward solution would just be IPC calls between writing processes communicating when a sym file is being written to, ensuring only one process writing at any given time but this should give you another option if concurrent writedowns is a requirement.