Tune the 'normal' strings first. Then tune the octave strings.

No need for a new tuner - your high D will still register as a D with the tuner. Same for the rest of 'em.

After you get everything tuned with the tuner, you should bump the octave strings, and the extra B and E strings, just a little bit sharp. This will cause a bit of a warble on chords, something like a chorus effect. It brings out the special magic that is a 12-string.