Doug, I have had Corsair to send me a new pair of my lifetime warranty mems a year ago, mems work in pairs, so if one is broken, you change both anyhow and one of my mems broke.
So, as thay work in pairs, or, on the new motherboards, as triples, you cannot run them in any other way, if you do that, your system will become very slow.
Also check the manual for your mbo or just look at the slots for the mems, it uses to be colourmarked for the slots that belong together, on my Gigabyte board, slot 1 and 3 is one pair whilst slot 2 and 4 is the other pair.
Mems on the same pair, should, as said, be of the same type and brand, best is to buy them in pairs and change both if one breaks.
hth