They are dual, but your clients may be trying to use 5ghz rather than 2.4. Turn off the 5 which will force them to use 2.4, and see if you get better range then.
I don't know that buying a "commercial"grade router will help, the Asus are pretty nice and the 2.5 and 5 ghz bands are limited by the FCC as to maximum transmission power anyway.
You could play with positioning, 802.11n can "bounce" around (multipath) resulting in better range. Try to get through doorways etc. Another possibility is to put them above/below - second floor or basement - so you're going through a ceiling or floor rather than a wall.
There are ubiquity on Amazon for sale: http://www.amazon.com/Ubiquiti-UniFi-Enterprise-System-Andnbsp/dp/B005SHQ644