I've known something like this: http://www.cycleexif.com/surly-dual-speed but I don't know this will work for casual bikes.
I also agree with @reiheit, internal geared hub is much more nicer idea. An example of this is bike rental system in Taipei (http://www.taipeitimes.com/News/taiwan/archives/2012/12/10/2003549807). I've been in Taipei for a while, and these bikes are awesome. They have 5 speeds (If my memory is correct.). Moreover, Internally geared hub is less prone to failures and I think the bikes require less maintenance.