Ok, to begin with I'm sorry about the limited amount of pictures taken during the actual build. I was a little bit too excited while building this to think of regularly taking pictures, some of you might know a little bit about this... So if the pictures don't make it clear enough, feel free to ask questions!
I've wanted to build a tandem for quite a long time but never really found the will power to do so... until I met my girlfriend! :D
I asked her "If I build a tandem, would you trust me and my creation enough to ride it with me?". The answer was "YES", so from that moment I was determined to build ourselves a tandem!
I scavenged two old bikes, one with a frame large enough for me and another with a smaller frame for mademoiselle. Oh, if you didn't get it yet, the idea was to cut and weld two bike frames together to make the tandem, and that's pretty much EXACTLY what I did.

Step 1: Cutting and Preparing the Frames

So, first of all, take off all the parts: pedals, chains, derailleurs, seat posts etc... From both frames. AND remove the fork of the frame that will be usesd as the rear part of the tandem.

FIRST CUT: You need to cut off the rear triangle of the frame that will be used in the front. I did this with a hacking saw and then smoothed everything out using a grinder.
SECOND CUT: Here, you only need to cut the head tube of the rear frame in half (vertically!) straight down the middle.

Then, once you have made these two simple cuts you need to find a way to align the two frames perfectly before welding them together. I did this by clamping two metal rails to the frames (see on pics). Once everything is in place (with the cut head tube of the rear frame set against the seat tube of the front frame) tack weld the two frames together. Now, before making the real weld pop the wheels back on and make sure that the two bottom-brackets are at an equal distance from the ground (roughly) and that that distance is AT LEAST 25 cm. If you're not able to do so, you'll have to figure out another way of welding your frames together or consider finding other frames. I was lucky and it all worked out fine; if it's your case as well, finish your weld (wich is easy because of the straight cuts!!) but keep the clamps and rails on.

Step 2: Reinforcing the Tandem Frame

You've now got a tandem frame! Probably a little weak and fragile though...
To remedy this problem I reinforce the frame by welding scrap piece of metal tubing, connecting the two bottom brackets.
Now, making the fish mouth cuts at the two ends of the tube is a little bit of a pain but its feasible! I just grinded away progressively until the length and the cuts were just right for the tube to fit in snugly between the two bottom-brackets.
Before welding this tube in place it's a good idea to remove all the bearings and axles from both bottom-brackets because the heat generated by the welding could damage the fragile bearings or melt plastic parts. Once this is done weld the tube in place.
Tadaaaa! The frame is completed, no more cutting or welding. It was easy wasn't it?
(Yes, I'm sorry, the frame is already painted in the pics. Just look at the tube that connects the two bottom brackets-together)

Step 3: Painting the Beast!

Here, you can go for anything you want, just make sure the frame is well sanded and that you use masking tape to paint only what you wanna paint!
The pictures are, I hope, clear enough for you guys to understand how I proceeded.

Step 4: Putting Everthing Back on Plus the Special Tandem Stuff. (1)

Now this is the looong and not so funny part.

On the rear (=stoker's) frame you can put the chain, rear and front derailleurs and right chainset back on. Now, on the left side, you're not going to put the normal crank arm back on, you need to find a chainset (which can be made up of only one chain ring) which has a ring of the same size as the chainset that bill be set up on the front frame. If you're lost, just check the pictures and annotations.

On the front frame you need to put the chainset (with a chain ring of the same size than the left-side chainset of the rear frame) on the left side of the frame (yes, it's the opposite to where it usually is).And put the crank arm on the right side.

The problem now is connecting the two "left-side" chainsets. As said before, you need two chain rings of the same size and aligned if you want to be able to pedal in sync with your partner. Putting the chain is not a problem, just connect two bike chains together to have a long enough chain (or rather one complete chain and part of a second one). The problem relies in tensioning the chain. What I did: I used a ghost ring to tension the chain, cause it's super good looking and super simple! If you don't know what a ghost ring is, check out the pictures or the video in the last step.

Step 5: Putting Everthing Back on Plus the Special Tandem Stuff. (2)

Now, once you've got all your chains, derailleurs, chainsets and pedals back on you can start fiddling with the brake and derailleur cables.
I re-attached the two cable guides on the bottom of the bottom-brackets. Connected two cables together to have a long enough cable to reach the rear derailleur and did the same to go up to the front derailleur. For the brakes, I reassure you, I DID NOT connect two cables to make it long enough. I bought a tandem brake cable long enough.

Sooooo, that's about it huh? Oh no, I'm forgetting one last thing, the stoker's handle-bars. What I did here was that a welded a little "double- clamp" from scrap pieces I had lying around, it clamps to the captain's set post and holds the handle-bars tight. You'll understand all this a little better with the pics hopefully and even if you don't, you can still ask me as maaaany questions as you want, I'll answer them with much pleasure!