How is my 2nd Airport Extreme Connecting To My 1st Airport Extreme?
SInce your second Airport is connected to the 1st one through an Ethernet cable, and if it isn't out of the normal length for an Ethernet cable I suggest you to choose this link only between your 2 access points (here Airport Extreme).

I advise you to configure the second Airport as follows:


suppress the option "Extend Wireless Network",
configure it in "bridge" mode,
configure it to create a wireless network with the same name and password as the one on your 1st Airport, but with a 2.4 GHz channel chosen so as to be separated from the 1st one channel by 3 empty channels, for example:
1st airport on channel 5, 2nd airport on channel 9 (this is a good practical example I am using to avoid the channels of my neighbours.

For 5 GHz channels just choose 2 differents ones (5 GHz channels don't overlap on each others as 2.4 GHz do).


With such a configuration you avoid to have 2 physical paths between your 2 AP which may cause flapping (packet 1 going path 1, packet 2 going path 2…) and loops (packet 1 going path 1, returned back on path 2…).