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.