Yes, you can make the second layer of a melt and pour soap another day. Simply allow the first layer to cool and harden completely, and then cover the soap with plastic wrap or another airtight covering to protect it from moisture or dust until you are ready to add the second layer.
When you are ready to add the second layer, remelt your soap base and pour it carefully over the first layer, being sure to avoid disturbing the first layer too much. It's important to pour the second layer while the first layer is still warm and slightly tacky, but not completely hardened, to ensure that the two layers will bond together properly.
Once you have poured the second layer, allow the soap to cool and harden completely before removing it from the mold. By taking your time and carefully layering your soap in stages, you can create beautiful and complex designs in your melt and pour soap projects.