Each reducer uses an OutputFormat to write records to. So that’s why you are getting a set of odd and even files per reducer. This is by design so that each reducer can perform writes in parallel.
If you want just a single odd and single even file, you’ll need to set mapred.reduce.tasks to 1. But performance will suffer, because all the mappers will be feeding into a single reducer.
Another option is to change the process the reads these files to accept multiple input files, or write a separate process that merges these files together.