Read your motherboard manual. There's probably 4 DIMM slots, correct? Using two side by side should indeed be only single channel while 1 and 3 is probably dual channel (mine it's 2 and 4). Not sure why having both sticks in one channel causes a worse OC tho... could be voltage, or some sort of imbalance/bottleneck.
I don't think there is such a thing as single channel RAM, as far as what an individual stick supports. Using the right DIMM slots will make it single or dual channel accordingly.