I assume you are talking about an 802.11n (MIMO) router. It is my understanding that in practical application, due to the number of 2.5 channels in use, that the only way to get dual channels and the higher bandwidth is to go to either 2.5 + 5 or 5 + 5.
The wiki is very good: http://en.wikipedia.org/wiki/IEEE_802.11
Yes, I kind of understood the basics. Thanks for the link it helped explain more.