If both sources are passive stages, then you can use a Join stage. Otherwise you have to use a Lookup stage. Read about both in the DataStage XE/390 Job Developer's Guide (which is in your DataStage documentation folder installed with your DataStage clients).
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But in the documentation say that stages (Join and Lookup) use one stage to the dataset and other stage to table but dont say it can be use to join 2 datasets ...