Biwiring is the process of feeding the different sections of the speaker crossover with separate cables (feeds) from the SAME amplifier channel. So your initial assessment of connecting the wires at the amp by twisting them together is about correct. You must observe polarities, and your speakers must support this.
The benefits are somewhat vague, with different opinions from those on each side of the arguement. As for the wire and guage requirements, the cable needs to be of sufficient size to not limit the output levels. A serious factor that comes into play is distance from speaker to amplifier.
