NAS systems are Linux based and don't use Windows file format. Try attaching one to Linux based machine and then running a File(Linux) recovery software.Check your NAS manual or call technical support for advice. Since you have 2 drives it makes the situation more complex. If you have a good, recent backup, you can restore the files from that. And if you don't have a good backup, the NAS manual and tech support don't help. If data is really important you can use data recovery services like Stellar to recover data.
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and your data is worth spending a lot of money, you can use a data recovery service like Ontrack or DriveSavers. Just make sure they know what type of NAS system contained these drives and whether they were part of a RAID.