Its just logical. You must only understand what transparency is.

Either you want overlay transparency that is invisible below 50% alpha, or you want alpha transparency that is still visible below 50% alpha. Naturally you cant have both at the same time.

Now you will ask "What is overlay transparency and what is alpha transparency"? Well thats why we have the Mighty Manual, just read the link that was posted as answer on your question.

In your case I would just use a transparent texture, TGA or DDS, and not an overlay texture.