The MVI56-WA-EIP must be installed in a chassis that also has a ControlLogix processor installed. It will not function in a chassis that is loaded only with I/O modules. Thus you can use two MVI56-WA-EIP modules to move data between two processors.
However, you cannot use a MVI56-WA-EIP module to bring in I/O data from a remote rack that does not have a ControlLogix CPU installed.