I got a BTK Leathers in Nottingham to put the extra zip from my fabric jacket into my leathers trousers so that I could wear either the leathers jacket or my fabric jacket. It worked fine and held when I had a minor off. I felt it was worth sending them to a bike gear tailor rather than the high street. It was several years ago but didn't cost much at all.
That said, I presume most kit bought as a pair has an all the way round zip whereas my extra one was only enough for across my lower back. I presume thats the compromise.