Mine out tiles with retaining walls

If you have a retaining wall, dig out next to it (so 5 down, lets say to the right), then place another wall below and to the right of it, and try to dig out that tile, it undermines the higher wall. It'd be great if it didn't work this way, and you could mine out those tiles, allowing another set of walls to be placed in the same tile.

I think the retaining walls break way too easily. And they really should be stackable. And they should snap to each other easily.
Currently, they are pretty much useless.