You can find more info on this link. P.S. You need Adobe Acrobat to read it.
It is a technical explanation of Dual memory application in fairly good layman's term.
Although they mention that the memory can be of different speed, I wouldn't advise it as your FSB will revert to the lowest of the speed.
Most boards will identify Dual DDR slots with color or position.
2 memory brackets OF THE SAME SIZE will need to be installed in the same channel (i.e. both brackets in ch. A or in ch. B)
In all cases anyway, your manual should show you the breakdown of your memory slots.
Hope this helps