The terminology "dual-channel memory" is being misused by some in the memory industry, which can mislead the consumer. The fact is there's no such thing as dual-channel memory. There are, however, dual-channel platforms.
When properly used, the term "dual channel" refers to the chipset on certain motherboards designed with two memory channels instead of one. The two channels handle memory-processing more efficiently by utilizing the theoretical bandwidth of the two modules, thus reducing system latencies, the timing delays that inherently occur with one memory module. For example, one controller reads and writes data while the second controller prepares for the next access, hence, eliminating the reset and setup delays that occur before one memory module can begin the read/write process all over again. Think of it like two relay runners. The first runner runs one leg while the second runner sets up and prepares to receive the baton smoothly and carry on the task at hand without delay.
If you have a dual-channel platform (or tri- or quad-channel) and you want to take advantage of the performance gain it offers, our advice is to simply purchase your memory in matchin sets. However, be very careful to order modules with the exact same specifications; the modules must be identical to each other to perform optimally.