This is how a simple opacity map works:

Rather than creating 2 different bitmaps, you could use an image file format which supports transparency information for each pixel in the image (TGA / PNG).

PNG texture:

Material editor.  Notice the opacity slot is not used:

Render:

Material editor – Notice how the texture bitmap image (PNG) is used both in the diffuse slot and the opacity slot:

Note that you have to change the “Mono(chromatic) Channel Output” to Alpha, so the PNG transparency (and not the PNG RGB) channel is used -only for those material maps that typically expect a black and white (monochromatic) bitmap; the diffuse map will use the RGB channel of the PNG image obviously.

Render: