Once we have managed to join pieces together to form blocks, we then treat those blocks as single units. We move them around like they are one piece, and we try not to break them up.
Treating these blocks as single units, we can once again use the align/join technique to join them.
If we rotate the front face 90 degrees anti-clockwise, the two blocks will be aligned and in a perfect position to be joined. Click play to watch the solution.