Is this a self join relationship or two TO's of the same relationship. Simplest would be to turn a field, or the whole row into a transparent button on the list view, and use a GTRR in the button setup
The OnRecordLoad trigger can be set up on the list view layout to perform a script that updates what record is shown in the other window. You can also select all the fields in the list view body and format them as a single button to perform this script such that clicking a row in the list view opens/updates the second window.
This can also be done all in one window and layout instead of two windows by using a portal for your "list view" portion of the interface.