An atlas file (I assume .txt here) can actually hold information for several textures/images (.png).
The scheme works like this:
Each Atlas holds Atlas Pages (individual textures)
each Atlas Page holds many Regions (individual packed images).
An Atlas can have only one Atlas Page, or it can have many.
When all the regions fit in one texture, you just have one Atlas Page.
Otherwise, it spills over and you get more than one page.
Here in your case, if you open BW_Button.txt, it should contain information for both BW_Button.png and BW_Button2.png.
If this is an incorrect description of your case, let us know.
