Tall Bike Buidling

Step 2 Cut and weld the front fork pieces.

Chop off the threaded portion of what used to be the forks to the top bike.
Drill four holes just below the threads.
Drill four holes into the fork tube just below the treads, on the bottom forks as well.
Insert the steel rod and weld it through the four holes.
Assemble the forks with bearings, nuts and all. Place all of the bike parts together and insert the rod through the top fork bracket.
The hard part here is estimating how much top rod to cut off. Mark where your top threaded piece needs to come out of the top fork bracket, estimate and mark your steel rod.
I made the mistake of not allowing for enough space to insert the handle bar assembly into the top of the threaded top fork assembly. Keep that in mind when making the top of the fork.
Cut the steel rod (preferably with a chop saw) at the correct length.
Weld the top of the steel rod to the top threaded piece. Make sure it is nice and straight.
Let it cool and then temporarily assemble the fork piece, while making sure the bottom of the bike sprocket housing for the top bike, lines up nicely with the seat post of the bottom bike.

Alternate design: My latest bike I created the drive shaft after welding the bike. I used a long rod to get the alignment. Having done both ways, I can't say which way is better.

Note: when welding, your frame will shrink a little, making the threads a little to long. This is hard to estimate for shrinkage.