Well first off, the sticks do not have to be identical. Only in size. It is recommended that you have the same freq, and timings. But most likely the board will adapt and use the standard of what ever is slower.
Also dual channel kits seems to be cheaper than buying 2 seperate sticks for instance. Sometimes it is the other way around, and the maker of the ram uses "dual channel" as an advertising element.
They do make a board that supports different size ram, and dual channel at the same time. I cannot recall the maker of the mobo. But it is possible.
INTEL i7 930
3x2gb Corsair Dominators
EVGA GTX 480