Actually, not only timing paramteres are crucial in Dual Channel. It's all up to the chips soldered on PCB - exactly it's up to their organization: 64x8, 128x8 etc. - then you've got one sided or two sided DRAM. That's why if you put on Dual Channel mode two modules with the same parameters, they will not always work us DCh. That's why most of DRAM vendors have special testing procedures to prepare Dual Channel kit.
Actually, to use 'dual channel' feature, both memory modules need to have exactly same specs else wise, both will be accessed with the lower specs. It is for this reason, the slots in the motherboards are color-coded.
This is a very interesting article.
I've always heard that we must not mix memory modules of different speed. Is this the reason why? Are these timing parameters the ones that should be matched when inserting more than one memory module in to a PC's motherboard?