The First Bar is for beginning an ambush. The second is for 1v1 melee combat. You can switch bars back and forth during larger battles to use a mix of ranged attacks.

The Ambush:
If you're in a duel first use Shadowy Disguise.

Hit your opponent with Shrouded Daggers and then immediately tackle with Ambush. Now that you're buffed, follow up with a Rend to lay down a major DoT on your opponent and a major HoT on yourself.

-Switch Bars-

Use Suprise Attack liberally in order to pumble your opponent while weaving in Debilitate every 8 seconds; it'll add another DoT to your opponent and it'll give you a movement advantage over them. It shouldn't take you too long to work up enough ultimate to hit your opponent with soul harvest. As soon as your able, start spamming killer's blade.

Dark Shades helps you with blockers. Magelight helps you find nightblades sneaking around in Cyrodil.