When the current bar gets small, get out a fresh bar, get them both wet, press the small bar firmly into the fresh bar, and then treat them as all one piece. There’s a chance they will come apart, so handle with care the first two or three showers. My parents always did this when I was growing up, and I was astonished to find out that some people don’t. It works best if both bars are the same kind, but they don’t absolutely have to be. If you bring home tiny hotel soaps, sometimes they can be attached to a new soap before they’ve even been used.