Using 2 pair of speaker cables for single hookup ?

You need to make sure you're speaker cables are all exactly the same length, or you will throw it way out of phase. The only affect having, say, 2 pairs of 8' speaker cables like that is a miniscule increase in efficiency of the power transfer thru. Also, you always want to keep your speaker cables as short as possible.

Why don't you try it, if it sounds better, then I would say that is a definite advantage.

The time that it takes a signal to travel down these cables is a fraction of a microsecond so I don't think you need to worry about phase shifts.

To answer your question, the only advantage might be a reduction in impedance. I don't recommend it, unless you are using identical cables the speakers will probably be recieving a "smeared" signal.