Dual band... is it simultaneous dual band? And does it allow you to have separate SSID's?
if so.. then it usually will for *bandwidth* purposes but *not* for the Loopback/wireless to wireless... the only way to know for sure on the bandwidth side, is to have one device that is G only, and a device N only and have them show you the bandwidth they are consuming at the same time ... its more complicated but you'd setup a load server via ethernet to your router.. and have both wireless G & wireless N devices access that load server
your wired computer could be the "Server" in respects... if you tried temporarily using an FTP server on it ... and then attached to your wired PC from your wireless G & N devices ... a regular "Windows file transfer" doesn't perform as well as FTP does ... there's overhead from windows that slows that type of stuff down ... but it might still give you an idea. Use a file like an ISO image ... like download a Live Linux CD to your wired computer.. then start the copy operation using the laptops.. (ie. don't CTRL+C on the wired computer and paste *to* the laptops ... get on the laptops connect back to the wired pc's hard drive and copy from it)
As for the separate wireless network .. no even though you have dual band and even if it treats them as separate wireless networks it most likely will not work for wireless to wireless device because of forwarding ... how to make one wireless device traverse the network. Not all routers have the problem .. nor all firmwares for those routers (I use DD-WRT on mine, and the newer versions have issues with traversal due to Loopback being disabled)
You could also be having a UPnP related issue in that some devices seem to not be able to cross properly from internal network to internal network when the device is setup with UPnP (or port forwarding) ... the router seems determined to either not forward wireless to UPnP devices or drops packets as some network oddity (again more of a loopback issue)
A good explanation of NAT Loopback first 3 paragraphs
NAT Loopback Routers - OpenSim
also noticed they list some of the 2wire routers as capable of Nat loopback and others not... which *could* be another reason this is happening for others with 2wire modems!
You should try hooking via Ethernet with the laptop (turn off the wireless on the laptop, don't leave it enabled while testing ... if you don't have a physical on off button, then the wireless symbol should be on one of the function keys you'd hold your Fn key down when you press that function key). If you're still unable with the laptop to connect to the 722 ... same error ... then some software on the laptops may be causing the problem... disable firewall, disable antivirus, disable *any* "downloader" applications you have..
Now ... the 722 .... L721 caused some form of change.. seeing that there may be a new issue with NAT Loopback maybe that's the issue, that they've changed the port structure from External is one port, Internal is a second one, to only 1 port and thus the local to local fails... purely open thoughts there, haven't looked yet for proof but it seems plausible.
In any case... when L721 came out the widest sudden rash of problems started occuring with Dish showing receivers "Off Line" and this problem has continued for the 722's (non-k) now that they are running L722 firmware.
these 722's almost makes me wish I had an effected one to test/play with to find the true source of the problem.. but keep plugin' away.. eventually something will cause the problem to be found and mitigated short term.. long term we need Dish to find and fix these issue..