Yes, you are able to use two or more texture packs. Given that they do not overlap the textures of the other pack. For example, you are using a texture pack which replaces only the diamond sword ; you can also use a texture pack which replaces the golden sword, iron sword and etc. If you use two texture packs which replaces the diamond sword, then the texture pack which is higher in the load order would be loaded.
You can only use two texture packs together if they do not overlap the textures that they replace. If you do so, it depends on the load order. Which ever is higher on the load order of your active packs list will be loaded.