Yes, you can do this, but.
- Check if your backups are up to date.
- Check if you can actually read these backups.
Both step 1 and 2 should not be needed. But play this safe and test these. - Pull one of the drives. Either via the GUI manager, from a running machine or during a power down.
- Rebuild the array. This should be automatic. With a mirror/RAID-1 this should be relative quick.
You now have a 64Gb drive with a 38GB (36?) filesystem. Next you want to expand the volume in the larger drive, or add a second volume. Else you have more space, but it will still be unused by the OS.