No, it doesn't use them uni-directionally. Just double the bandwidth in the true sense of the word.
Unusual in the UK for it to be defaulting to 40MHz, which gives/requires dual channel. I used to use it.
It often gives you a higher wireless connection speed to the router from your kit, depending on the quality of the wifi card in that kit, but is regarded as neighbour-unfriendly as you interfere with many more channels. It doesn't necessarily give you any higher throughput though.
20MHz will give you a single channel.
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Edited by RobertoS (Sun 09-Feb-14 12:51:43)