Why use Channel Packer:
Channel Packer allows you to pack up to 4 different images in one texture, to manage and process them at the same time, saving memory.

Channel Packer works in two different ways:
- Single mode: used to generate a single texture, packing up to 4 input images;
- Batch mode: used to generate many textures in a unique operation, starting from a list of images; it allows to associate them to the needed channels and pack them in a texture for each association.

Further options:
You can invert images, use colors, select channels to use, and more.

It does NOT require Unity Pro

Read the full documentation.
Official forum thread