Just ran into the same issue last night. I think it is not possible right now as paving the flatten ground breaks the flattening job in place and you can only lower ground after the pavement is placed. Just make the path 2 tiles wide to make it look better... if possible.
This should be something to be corrected in the future...
I've been paving roads for the last week and I don't even waste time "flattening" anymore. I'll use the "Flatten Ground" action, but as soon as I see the progress bar crawl (meaning that it's just making the appearance flat and not changing the ground level) I hit ESC and move onto the next tile.