Two Airport Expresses extending the same network and a weekly password rotation?
If AE1 (plugged into the router) is setup as a wireless access point, then put a WPA/WPA2 password on that wireless network.

If AE2 is setup via Airport utility to "extend" the wireless network of AE1, then it will automatically inherit the password setup from the primary (in this case AE1).  Any password changes you make to AE1 will be transparent and update across both devices.

If you have setup AE2 to "join" a wireless network, this will not happen.

With "extend" the end user will only see one wireless network, with the same name, not two different devices.  Unless you have set your network up differently.