Multiple sticks of memory will always run at the lowest common denominator. They will both run a CL3 and have the timings of the lower grade memory.
Using mixed sticks, especially chips, is never a wise idea in dual-channel. The memory must run in pefect parity or you'll get a ton of memory errors and BSoD's.
I'd say run those sticks in single channel mode, it'll still run at the lowest common denominator, but at least you get the extra memory which will make the biggest effect.
If you want a error proofed (mostly) experience with dual-channel, buy a matched pair dual-channel kit.