Note: This Tutorial was written for 40d, but it still works with DF2010.
How to break through two layers of aquifer with caveins.
Forum Thread: http://www.bay12forums.com/smf/index.php?topic=31651.0
Click here for image: 
Movie Part 1-2: http://mkv25.net/dfma/movie-1138-howtoaquifiercavein
pull the lever! i am sry for the real long responding time as you see on my fps, this was really shorter then i did it ^^
Part 2: http://mkv25.net/dfma/map-4854-steelmarket
the aftermath ^^
Part 3: http://mkv25.net/dfma/map-4855-steelmarket
and the second cave in is ready to go off!
Movie Part 3-4: http://mkv25.net/dfma/movie-1139-howtoaquifiercavein2
again some lever action
Part 4: http://mkv25.net/dfma/map-4856-steelmarket
the cave in is done
after the clean up.. stone! dwarven orgasm.. well right after booze