The suggested method is probably easiest with bath soap, where one uses a facecloth or sponge (or the net equivalent), although it works with smaller bars, just as well. By rubbing the sponge consistently on one side of the soap bar only, that side wears down quicker than the other, becoming concave, making it easier to ensure that the thin, remaining sliver of soap (from the almost used-up bar) fits snugly against either convex surface of a new bar.
Pressing them firmly together when both are wet results in what becomes a single, homogeneous convenient-to-use bar. If the two bars are the same color, they soon become indistinguishable as being joined components and the soap is used to the utmost benefit. What is more, your guest bathroom will not be disgraced.
And there is absolutely no waste, whatsoever.
Related: 
Using Leftover Pieces of Soap
By
11/16/2010
I make my own lye soap for bath and laundry. I use left over bar soap. I stay in a lot of hotels and the soap from there I use also. I take a 5 gallon bucket that has my lye soap in it, I continually add the soap bars to it and fill it up with water. When the bars "melt" I run them through a strainer and use as laundry soap. I haven't bought laundry soap in over a year!
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