Gradient Alpha Mask

I have done several searches on how to create a revealing gradient mask. I dunno if I’m just not describing the effect in which I want to create properly as I haven’t found anything specifically covering this topic. Sure I can find tutorials on how to do a “glint/shine” effect. However What I want is to have to similar images with an alpha gradient mask between the two images, and when the mask passes over; it reveals the image below with an alpha fading gradient on the edges of the mask. I can get the effect, with the exception of the alpha faded edge; its just a mask with a solid edge. This IS possible to do, . . right?? As I’m sure that I’ve seen something similar to what I’ve described here. If anybody can help or point me in the right direction on where to find help on how this is done; I’d appreciate it. thanx, -ssf-