Do alpha cut off with seperate texture?

My situation is as follows: I have a texture with transparencies built in, that is transparencies I want to have independent of the alpha cut off value.

Is there a way I can do this? I’m thinking if I could use a different texture for thea ctual alpha cut off calculation it should work but I’m not sure how to write a shader which would do that…

There is a shader that does precisely what you want at the wiki.
http://wiki.unity3d.com/index.php?title=Dissolve_With_Texture