No matter what, you need more than one taillight. WHEN your taillight fails, you won't know it, so you want to be running two.

Actually with the higher power lights (Dinotte, MagicShine, Hotshot) I can look in my mirror and see traffic signs 300 feet behind me getting lit up by the lights flashing, so I can tell if they are on or not, but I don't know immediately if they fail.