The ipad 2 uses only 2.4ghz .. the later ones can use either 2.4 or 5ghz.
If you leave the ipad 4 off does that make any difference to the dark areas?
Did the iOS upgrade result in any changes.. ? although you say nothing changed I think you will find there are wireless issues on the latest OS. But I am not an ipader.. so you will need to look it up.
What I would do.
Reset the TC to factory and start again.
Use SMB names, for TC, wireless and hard disk. (to be thorough). ie short, no spaces, pure alphanumeric.
Give a different name to the 5ghz wireless.
Use the radio channels control to fix 2.4ghz and test channels 1, 6, 11 and stick with the best one.
5ghz you can leave auto.
Use WPA2 Personal security as the only security which is designed for N wireless.
I have tried LaPastenague's suggestion of establishing a specific channel for the 2.4ghz, it didn't help in all areas of the house. Unfortunately the problem may be specific to that iPad2. My wife's old 1st gen iPad doesn't lose the network in any part of the house no matter where it is relative to her 4th gen iPad. When I get mine within a ten foot range of the TC or my Airport Extreme bridge, it will connect to my network. The other devices, my MacBook Pro, my wife's MaacBook Pro, iPhones etc. have no problem accessing the network.
It's a week and half later and applying every know software and firmware work around still the problem exists. I went to the Genius Bar and had the iPad2 diagnosed. Th short version is that they believe i have a faulty wifi antenna. Not sure why it's faulty now, no abuse, in a case and it hasn't been dropped. They can't fix it. My alternatives are turn in the iPad2 pay them another $249 and get another iPad2 (which may or may not have the same problem); or recycle it and get a $225 gift card and apply it to a new iPad that costs $699 (if I want Retina display). Either way I feel like I'm being skeddicked. I don't know for some reason I believed I would actually have a device that would last more then 2 years. I went back to the Apple store to see if I could turn in the old one , buy a new iPad and automatically get the $225 recycle fee applied. Oh no, I would have to ship back the iPad2 get a gift card and then order a new iPad online. Major fail at all levels on this one. So ends my rant thank you for listening.