See addtextures1.jpg - put your cursor on A, hold down mouse button and pull it to B. This will leave a blank space as in addtextures2.jpg. Then click on the texture you want to add and click the + button. Don't double-click to add it as that will add the texture to the red section.
Pictures are from an earlier version of BTB, so the layout may be a bit different in the latest version.
edit: In v0.800 right click on the space you've created and choose 'Add Material'