By way of backup, do you want to just backup selected files (e.g. documents, photos, music, videos, email etc.) or do you want to backup the complete system (e.g. make an image of one or both of the disks?
In any event, how are the disks currently utilised. E.g. is each just a single partition or what? And what is stored on each disk / partition?
Tied in with this, what medium do you want to backup to? One of the disks to the other or to an external disk or pen drive or...?