The dual channel will be automatically enabled with 2 sticks of ram... On my Asus board, its recommended to put them in slots 1 and 3.
"Dual Pack" or "TwinX (Corsair)" models are paired sticks guranteed to run at dual channel memory at higher rated timings. Corsair tests the two sticks together to ensure spec level performance and timings, and will RAM both sticks if any problems occur. Buying two separate sticks runs the risk of having one perform better or worse, each scenario requiring separate RMAs or other performance issues... Though for the most part, two good sticks of ram (ones that perform well by themselves) shouldn't have any problems running together. There are some who believe that dual/twin packs are a marketing gimmick, but I personally think the Corsair testing and QA for the twinX is worth the price premium.