Enable product-mixing to read from multiple secondary input files per primary event
Issue #21384 discusses a scenario where an infinite loop can be entered due to inadequate checking during the
generateEventSequence. Although the bug of entering an infinite loop has been fixed, the product-mixing system does not support reading secondary events from multiple secondary input files for the same primary event. This issue requests the feature/bug fix that multiple secondary input files can be used for product mixing for a given event, thus fixing a limitation of the system.