The first dual channel DDR chipset for the AMD Athlon platform was released by nVidia, named "nForce". Instead of having a single 64-bit controller, the nForce has two 64-bit controllers which allow for twice the memory bandwidth. To make use of this, however, you need to be using two 64-bit memory modules, which, to me, is quite logical. However, again, the extra memory bandwidth is not used by the AMD Athlon because it can only provide 2.1GB/sec bandwidth, rather than the 4.2GB/sec that the nForce can theoretically provide. This is why the nForce chipset did not provide any extra performance over VIA's KT266A chipset, unlike what was originally expected.
However, if nVidia decided to base their nForce chipset around the Pentium 4 platform, things would, again, be much different. The Pentium 4 can provide up to 3.2GB/sec of bandwidth, which is about 40% higher than the Athlon's 2.1GB/sec. The reason why nVidia decided to go with the Athlon platform rather than the Pentium 4 was most probably due to licensing issues with Intel.