I first load the first film the normal way but I keep on winding the reel until the film goes a quarter of the reels diameter past the reels start. Then, I put the second film through the reels start and wind it until it touches the first films end. I tape them together with a small piece (around 1-1,5 cm) of electrical insulating tape..
If you're trying to process 2 rolls on 1 reel, possibly use a 120 reel and load the film with the base facing each other and the emulsion facing out. I've loaded 35mm this fashion and it works. However, you have to wash the film off the reel.
I've never tried taping them together but I do routinely develop two 120s on a single Paterson spiral. Load the first one and then carefully push it onto the spiral as far as it will go, then load the second. Load it until the end is just past the ball bearing. Never had a problem.
I really screwed this one up tonight. I loaded the first roll but did not push it forward like it clearly states. For some reason that was perfectly clear in the dark, I said, "hey, if I just fold the tape over on the second roll, it will push the first roll along to the end" which it does. But, it also has a habit of causing the second roll to jump over a portion of the first roll as it pushes along. So I now have one roll that has undeveloped portions on the last two negatives, and one roll that has undeveloped portions on the first two negatives.
As Murphy says, if you know what you are confident in what you are doing, then you are doing it wrong.
Once a photographer is convinced that the camera can lie and that, strictly speaking, the vast majority of photographs are "camera lies," inasmuch as they tell only part of a story or tell it in a distorted form, half the battle is won. Once he has conceded that photography is not a "naturalistic" medium of rendition and that striving for "naturalism" in a photograph is futile, he can turn his attention to using a camera to make more effective pictures.
I often load two rolls togerther. The trick is getting the edges aligned perfectly and not having a gap between the two films. I use the piece of tape which is already on the fil attaching it to the backing paper.