Dual Channel memory access is a technique where the computer’s memory controller can access two identical memory modules as if they were one large module. This gives a theoretical doubling of bandwidth between the memory controller and the memory. In real life, it translates to about a 6% to 8% speed increase over the same amount of memory in Single Channel mode.
Dual channel memory access (and triple channel for certain desktop machines) is dependent on the motherboard having the functionality. It cannot be added to a machine that does not support it. Most machines with DDR2 and DDR3 memory do support Dual Channel, some machines with DDR (PC3200 and PC2700) memory do as well.
There is no difference in the memory modules in a Dual Channel configuration. Some manufacturers sell pairs of memory in Dual Channel sets, all this means is that they are guaranteeing the two individual modules are going to match.
You don’t necessarily need to buy the two modules for Dual Channel at the same time, or even of the same brand. The requirement is that the modules have to match in their size, their speed, and their composition (how the memory chips are organized on the module and the logical ranking of the rows and columns of memory). So it is possible to have two modules from different brands make a successful Dual Channel pair.
Size and speed are easy. The catch is that it is difficult for you to determine the internal organization of a module to know whether the composition is the same – so the easiest way to guarantee a Dual Channel pair is still to buy two modules of the same make, model and batch.