If you're like us and Time Machine just isn't enough, you can RAID the two drives together to make a single 1 TB drive.

Instructions are elsewhere online to do this, but the basic steps are:

Back any data up, then boot off a different startup disc.

Format both drives as Mac OS Extended.

Create a new 'RAID set', and add both drives to it. You can choose 'Concatenated Set', mirrored RAID 1, or striped RAID 0.

After you create the drive array, you can copy an OS onto it from a backup (as we did), or you can install OS X from scratch.