It is not actually a secondary class. When you reach level 15 you are able to equip a back-up weapon that you can switch too by tapping the left arrow. When you open your skills after equipping this secondary weapon you are granted another ability bar, but you still use the skills you have unlocked with leveling up.

If you read through the class builds on forums, you will notice many have a setup for both bars along with ideas for weapons and skills. Personally I think the best use of this is to give yourself options in a fight. For example, my nightblade is using dual wielding with single target dps in my first bar. In my second bar I have a bow with crowd control/debuff type abilities with some damage thrown in (poison arrow and volley ATM). This gives me the opportunity to switch from direct memes combat to ranged/kiting almost instantly.

Also, another handy thing to keep in mind is that when you are turning in quests or gaining experience from fighting, the abilities on the bar you currently have equipped are the only ones that gain experience. If your not comfortable using a new type of weapon just yet for big fights put that weapon on your secondary bar with other abilities you want to level up and then switch to that bar before you turn in quests. Great way to level abilities without using them all the time.

The when you open your inventory after reaching level 15 there will be a new slot for main hand and off hand back up weapon. Just equip something there. When you open your skill menu (where you go when you use skill points and to assign skills) you can use triangle to switch back and forth between your weapons and the bar that goes with them. Switch to your new bar and assign abilities to it from there.