the difference is there are 2 phyiscaly separate loops in a dual loop, and obviously one loop in a single loop.
people go with dual loops because there are many blocks in the loop, too much for a single radiator to cool, so for example, if you had 3 GTX 480s your motherboard and a processor, that is an isntance where you would want to go dual loops... so the graphics cards on one loop, and the motherboard and processor on another loop.
cool?