Yes, you can. Duals are not available in regular town stores, you have to either get the blacksmith to make you some (buy two D grade swords and then get them crafted together, which costs a medium sized fee), or buy them from a private store. Top D grade duals are called 107s, and can be very expensive for a new player. There are many, many duals to choose from-- Press the Duals Tab (http://www.lineage2.com/Knowledge/item.html), and then scroll down until you see D grade duals.
As a Palus Knight, I also recommend trying for a two-handed sword. I have seen and heard of many Palus Knights who use two-handed swords pre-40 and feel that it was the most efficient of weapon choices.