How to Build a Tall Bike, one that you can stop safely, and could be construed as street legal in most places. With this method, you'll be able to stand over the bike, allowing you to hop down easily, have two brakes, a full complement of gears, And have a better riding position than two bikes stacked on top of eachother. You'll only destroy one bike, The top portion is bolted to the bottom bike, so if it breaks, or you get tired of it, the bottom bike can be reverted to normal. All in all I think this makes for a suprisingly sane tall bike. Not that you'll look any less the madman riding it through traffic.
Step 1: Gather two bikes, extra bits
You'll need a pretty functional bike for the bottom, and a bike for the top that just needs a good bottom bracket, and a frame that isn't rusted away. I prefer cheapish old mountain bikes.
As far as parts;
"steel seatpost to fit the bottom bike
"full length brake housings/ one normal cable front, extra long cable rear
"handlebar/stem cobo to reach 28-32"
If you want to get multiple gears running you'll need
"two farly well matching drive side cranks
"extra long gear cables
Step 2: Strip the top bike of parts
first, to make things easier, you're gonna want to strip all components off the back half of your top bike, cranks, derailleurs, bottom bracket, brakes, seatpost, back wheel
Step 3: Cut the rear trriangle off
Now, our first cut, slice the down tube and top tube off flush with the seat tube, leaving a nice clean rear triangle.