Step 1: Our target
Step 2: Supplies
Step 3: Ripping things to pieces
BACK BIKE: take the handlebars and front fork off the bike that is going to become the back of the tandem. Tear out the cables too, as we won't be needing them. Cut of the gear-changer for the pedal sprockets, as it will get in the way later. Now remove the bearing races from the head-tube (the 'grooved washers' the ball-bearings that support the front fork ran in) by hitting them with the hammer. (the bearing races are just a press-fit over the tube) Now hacksaw through this head-tube as shown in the picture. (we're trying to make a 'cup' that will cup around the seat-post tube of the front bike) That's our back half done!
FRONT BIKE: remove the rear wheel and all gear-changing equipment. Now hacksaw off the support tubes for the rear wheel as shown in the second image - we want the lower tubes left on but without the wheel brackets and we want the upper tubes completely gone.
You should end up with something looking like the third image if you put the two next to each other.