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Is is possible by using the QNAP package for Syncrify to backup 2 QNAP device between each other?
Yes. You can run Syncrify Server on one QNAP and Syncrify Client on the other. Since QNAP does not have GUI, you will have to use the Command Line version of Syncrify Client. Check http://web.synametrics.com/syncrifycommandline.htm for details.