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.