I have a Linksys E4200 dual-band router. In fact, I replaced it before coming to the Intel forum to make sure that it wasn't router hardware causing the following issue.
My 2 GHZ WAP gets 65Mbp/s to its connected devices that do not have the Intel chipsets. However, the 5.4 GHZ WAP gets 48 to 54 Mbp/s in a fluctuating manner. I only have two Intel chips and the two devices using them are what connect to the 5.4 GHZ WAP.
The two laptops operate with Windows 7 and Ubuntu 11.10. Both operating systems have the same results for connection speeds. The WiFi cards are Intel Centrino Advanced-N 6200 and 6230. Both fluctuate at 48 - 54Mb/s on the the 5.4GHZ channel.
If I put the 5.4GHZ on Wireless-N only, the laptops cannot connect; it has to be on MIXED (A & N). I have tried AUTO detection for mhz and channels as well as 40mhz with channels 149 and 161 with no difference.
Currently, I am operating both WAPs on different SSIDs to make sure my results are separate.
Security for both WAPs is WPA2-Personal with a 10 digit sequence.
When using a dual band router you can choose to use the 2.4GHz band for old 802.11G devices only and the 5GHz band to wireless N devices.
In your case since one computer have the Intel® Centrino® Advanced-N 6200 and the other the Intel® Centrino® Advanced-N 6230 you can setup the adapters to use the Wireless-N connection with the 5GHz band and it that way use the band that has less interference to get the higher speeds.
One good thing is to have the latest version for the Intel drivers, for both computers the latest version is the 15.1.0
The latest version for Windows* 7 are available at: