Hold two squares together so that the loops are offset, matching a “bump” with a “dimple” so that they will be nested together when joined.
Thread seaming yarn on a tapestry needle. Pass the threaded needle through the corner loop of both layers from front to back, then pass through again to secure.
Pass from the wrong side (inside) to the right side (outside) of one square through one half of an edge loop, then pass back to the wrong side through the second half of the loop. Repeat with the other square, moving back and forth between the squares in a zigzag.