This is an old newspaper photographer's trick. The objective was to have a way to process film as quickly as possible, and this cut the time in half. And it was often coupled with the use of 12-exposure 'press' rolls that allowed a photographer to dedicate an entire roll of negatives to one event (fire, accident, grip & grin, etc), which meant that there were often more rolls to process to get a day's worth of shooting. Since the idea was process and print in time for tomorrow morning's edition, and no one cared much about the archival properties of the negatives, it was perfectly OK.
I've done it once or twice, but I've always been concerned about two factors. One is exhaustion of the developer - after all, the method means that there is twice the surface area of emulsion to be processed. Using fresh developer in a one-shot sequence always seemed to work for me, but I would be very cautious about using partially used or replenished developer. The other concern was that it was difficult for liquids to get between the two strips of film. That's not a problem with development and fixing, but I was concerned about washing - I could envision a situation where enough chemical got between the layers, and didn't get rinsed out, to leave the back of the film contaminated. So I always manually rinsed each strip of film individually to make sure that the back was clean.