You can achieve several effects through layering. Layering might entail any of the following: two color layers, fused together; two separate textures joined with a third, contrasting layer; and fancy slanted patterning. Layering is done in at least two steps.
In a half-and-half layer pattern, you make and pour one half of the volume of the mold. Let the first half sit long enough so that the second layer will sit on top of it. Then, make and pour the second layer, which is the other half of the volume. The layers bond and fuse through the heat of the insulation period.
When you're working with three or more layers, you'll need to make and pour the layers at higher temperatures to be sure that the soap will go through the gel phase. It may take some experimenting with times to find what works best for your climate and ambient temperature.
You can tilt the mold one way for the first layer, then another way for the following layers for an angled effect in the cut bars. Be sure you don't tip the mold so far that the soap pours out. Also, make sure that before you tilt for the next layer that the first one is totally firm.