Communicating offline between different android devices
This is fairly simple actually. With "ssh server" (ice cold apps is a good one), you can install it on each android device, and then connect from one device to another across your Lan. Of course if you have no network connectivity you won't be able access the Web, but it should be possible to push pull data using rsync (you will need busy box and root to gain full functionality), but it is certainly very possible.

Although, depending on what you actually want to achieve, some easier options are available - eg for simple file sync, you can use bittorrent sync, even without an Internet connection