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Got a question from my brother and I'm having a hard time finding an answer to this specific question.

My brother has a Haswell i5 and a Gigabyte Z97MX-Gaming5. One kit of G.Skill 2x4GB DDR3-2133 and one kit G.Skill 2x8GB DDR3-1866.

He was having some issues over the weekend where his PC wouldn't even hardly post. Long story short, it seems that a BIOS flash did the trick (not sure why). In the meantime we got to discussing his RAM speeds/timings/voltages (since I was convinced RAM was the source of his problems). I told him he could probably run them all at 1866MHz CAS 9 @ 1.6V and be ok. But he noticed a setting in his BIOS where it seemed he could set different speeds for each bank (A slots and B slots). I've never heard of this.

Is it possible to run each kit at different speeds?

easy answer... nope.
It will run at the slowest speed.

Little more info, after I'm skimming through the manual for his board. In the M.I.T - Advanced Memory Settings there is an option for "Channel A Memory Sub Timings" and Channel B Memory Sub Timings". I believe that's what he was referring to. It seems the frequency has to be the same between both channels, but you can set different sub timings (CAS, tRCD, tRP, tRAS, etc) for each channel. Not sure what the benefit of this is, and/or if it's recommended to run each channel on different sub timings.

If that option exists, the memory that's forced to run at a lower speed to match the rest could have its timings tightened up.

However, I have no idea if this would help anything, and, in fact, Intel Core architecture doesn't seem to care about memory speed very much.

Based on reviews I've read, Haswell-based systems see about a 1% overall speed difference per 2 memory speed classes, and that's also affected a little bit by CAS timing as well.

The impact of running the faster RAM at the lower speed might not even be measurable.